Different fight organizations have different medical standards but since you mentioned that you're basing this on the UFC, we'll go with UFC rules.
The UFC is notoriously stringent in their medical standards because they like to market themselves as the safest sport organization ever.
"Here's the reality. This is the most regulated sport in the world," UFC president Dana White told the UFC 128 pre-fight news conference.
There are 3 medical tests a fighter must undergo to fight in the UFC:
Medical test 1:
MMA combatants must undergo annual tests to maintain their licence to fight.
Medical test 2 (prefight):
They also have a medical test when fighters are chosen for the card (this can happen 3-5 months before). The medical test includes: a check up by doctors sanctioned by the State Athletic Commission (which state/country depends on where the UFC show will be held. A San Jose show would mean the California State), CT scans, X-rays and MRIs if they spot major anomalies.
Medical test 3 (right before weigh ins):
Then another test happens right before the weighs-ins to make sure a fighter did not injure themselves during fight camp but this test isn't as stringent as the one before. It usually just involves a doctor.
Medical test 4 (post fight):
Then after the fight, the State Athletic Commission imposes post-fight medical suspensions, varying on the degree of damage taken. "If they take any damage to the head, they're put on a three-month suspension," said White. "These guys are on a three-month medical suspension and cannot return until they're seen by a doctor.
Here are some examples of medical suspensions from UFC on FOX 12 "Lawler vs Brown":
Matt Brown: Suspended 180 days for possible fracture to open nasal area, Additional suspension 60 days for laceration to nasal area, unless cleared by physician. Minimum suspension 45 days (30 days no contact).
Robbie Lawler: Suspended seven days (precautionary reasons).
Antonio Rogerio Nogueira: Suspended 60 days for laceration to right eyebrow, unless cleared by physician. Minimum suspension 45 days (30 days no contact).
Anthony Johnson: Suspended seven days (precautionary reasons).
Clay Guida: Suspended 60 days for facial laceration, unless cleared by physician. Minimum suspension 45 days (30 days no contact).
Fighters often return to action much quicker once doctors give them the green light. The lengthy suspensions are just a precaution in most cases. In addition, not all injuries are recorded during the post-fight examination and often times surface in follow-up tests.
In your above scenario, it is very unlikely that a concussion that causes headaches and dizziness and whatever a month or two afterward will not be caught by a CT scan during the prefight testing. So in the above scenario, unless her fight camp decided to 'fake' her CT scan by switching out her CT scan with another healthy person's scan (i've heard of this happening before in other fight organization just not in the UFC), her concussion will likely be caught.
If she also sustained the injury during a fight, the post fight examination may catch it depending on how bad it is, this would also put her on the 'watch' list meaning that doctors would be on the lookout for it knowing she had sustained a concussion in her previous fights. Dana White says in his keynote address Jan 2013:
""Concussion is a huge dilemma right now for the NFL. Here's the difference between the UFC and the NFL as far as concussions are concerned. First of all, if you get a concussion, if you get knocked out or you get hurt whatsoever in the UFC, three months suspension. You are on suspension for three months and you cannot come back until you are cleared by a doctor. You can't have any contact whatsoever."
The UFC is really stringent with injury, they've cancelled fights before for the smallest things esp if the fighters are headlining the card. Given that female MMA is the new hottest thing, the females are under a lot more scrutiny then the males and hence the medical tests will be a lot more stringent I'm guessing.
Hope this helps!