Not getting sexually aroused is actually really common for women. Many will pretend they are to try to make their man happy (I have done that many times...although I am able to get "hot" and climax when I am in the mood). like damaged said some women just dont want to have sex or are bored with it. Perhaps she is not ready. Sexual pleasure comes with experience. Several years ago I was VERY shy about sex, only doing it to keep my man happy. I have been with him 3 years now and as I have become more comfortable we have been trying new things. She may just need time. Also if you are going to experiment make sure she is comfotable with it beforehand. I would try porn of some sort. Naked chicks are WAY hotter than naked men... seeing a hot naked girl always turns me on... and no I am not lesbian or bi.

I definately recommend seeing a docotor about the irregular periods though. It could be hormones, but it also could be something more serious. Better to stay on the safe side there...