So my girl friend and I have been going out for about nine months now, and I have noticed that she has an unusual menstrual cycle. She had on a fairly regularly basis. But it seems that latley she has been having her period about once every two-three months. We have been having sex on a regular basis, with condoms, the couple of times that we did have sex without condoms we made sure that she got the morning after pill.
p.s. she has/had a small milignent tumor on the base of her brain for a while. Any ideas what might be happining??

No idea what's going on, only a doctor can determine that.

On a side note, every time you have sex the possibility of pregnancy is there, even with a condom, even with the morning after pill. No form of birth control is 100%, no form of emergency birth control is either.

Get her to go to the doctor, ask her to find out if the birth control pill is a possibility for her, it would lower the odds of her becoming pregnant and it would regulate her periods.

Good luck.