Periods Skipping Every Other Month

My periods have always been a little crazy. But I have always had them and never skipped a month, unless of course I was pregnant. But since December, it seems I have skipped every other month. I don't know what is causing this. I'm not pregnant, I have taken a few tests since December. I even went to the doctor in January and the test came back negative. They couldn't tell me why my periods were being so weird, so maybe someone else can. I skipped December, had a 2-day (not normal) period in January, skipped February, had a 5-day period in March, and here it is April and haven't started yet, which would make me late according to the last periods I have had. I am sexually active and we don't use protection, but all the tests come back negative, so I doubt that I'm pregnant, tho I haven't tested this month. The clinic I go to isn't very helpful and its hard to make an appointment with a doctor in my town, because they don't accept my insurance, I would have to go to the ER and get a referral, I'm hoping to avoid that route. Can someone please help me figure out whats going on?