Point is that a real woman knows when something's not "right" with her period.
I had the same issues, my period had changed, was not as heavy anymore when I was about 23 or 24.
I went to my ObGyn and he simply did a blood test to check my hormones.
There is a certain hormone in your blood when you ovulate (I am not sure what the english name for it is). So he could tell me after this blood test that I wasn't ovulating. Ultrasound didn't show anything abnormal so I went back a month later to get my blood checked again to see if this was a one time thing about not ovulating, or if I never ovulate.
The next month there was no ovulation again.
I started a special diet a couple of months later as I have a thyroid disease and my hormones are fu**ed up anyways. This diet was supposed to get my thyroid gland and my metabolism to start working normally again and about 3 month later I ovulated.
I just knew it the day I got my period, cause it was really heavy and with blood clots and everything. Went to the doctor for a test and he said yes, I ovulated.
So now one month I ovulate, another one I don't. I ovulate maybe 3 or 4 times a year, but still get my period every month. Sometimes lighter, sometimes heavier.
As a woman you know when something is wrong, my doctor wouldn't believe me at first either, but well............I knew better.
Oh yeah, to the topic: I do not take the pill, sicne my hormones are messed up enough (that could be the reason for my bad mood swings), but when I do have sex I use condoms. Not because I have to fear getting pregnant, but to protect myself from STDs.
My periods were almost non-existent for a few years. Like Chantillylace I saw a doctor about it (in my case it was a GP - that's who you see here unless they refer you to a specialist). My doctor didn't seem the slightest bit bothered about it. But I was bothered! If I'm not menstruating it's a big red light saying that my fertility isn't 100%. I asked her to send me for an ultrasound to check for PCOS. Nothing unusual could be seen, and after that I was stumped - I had no idea what to suggest next, and my doctor didn't suggest anything either (because she thought it was 'normal'

).
Strangely, the problem solved itself. When I started having sex again (after a few years of no sex), my periods normalised again. It seems as though my body as reacted to the renewed sexual activity by saying 'right, best get ready for baby making then'. Very, very odd.
Chantillylace, having been there, I know it can be hard when medical people aren't being helpful, but this is something important that you need to look into. If you leave it until you have a 'donor', you may find it's too late.