It's totally normal to gain. Try your best to stay hydrated and you can try and minimize the effects of bit. It also seems that people put on the weight at different times in their cycle and that it comes off at different times. You'll get used to your body and start to know what to expect.
But I strongly agree with rockinrobin that you should see your doctor. If you aren't getting a period every couple of months it can put you at greater risk for a lot of problems later in life. It's definitely something you want to get checked out. I have irregular periods too as a result of a condition I have and thus, if I don't get a period every two months I induce it. It's really something you should get addressed.
Sorry...I know that wasn't part of your question and I hope I didn't overstep my boundaries.
