As you have done a pregnancy test, then I take it that there is a possibility of a pregnancy.
You may be pregnant and this was a threatened miscarriage. The test may have been done too soon.
It may be just what AnnieBrooke says.
You may be anaemic or your hormone levels are out of balance.
It is too early to say at this point in time what may be the cause.
Polycystic ovaries can also cause erratic periods. Anxiety, stress, extreme dieting are other possible causes of erratic periods.
I have never had an ovarian cyst, but I have experienced the other things I have mentioned (apart from the STDs). My daughter suffers from polycystic ovaries and can go for months without having a period and not be pregnant.
If you have been sexually active without using precautions like condoms, get checked out for chlamydia or other STDs. It is always best to air on the side of caution.
I suggest you keep a written record of this period and see what happens over the next couple of months. If your periods have come back to their pattern (if you are not pregnant), then book in to see your doctor to get some blood tests done.
Best wishes.
Sometimes that happens when you have an ovarian cyst. One time when I got one, my doctor looked at me and said, "Was your last period kind of weird?" and it had been.