My last two cycles have been very light the first month it was almost nothing, the 2ed month it was a little more but still almost nothing, I'm 24 and this has never happened before, I did have inercorse the week before I was going to start and that's when the first one was messed up, so I was thinking I might be pregnant, but the tests come back negative, I talked to my mom about it, and she said her tests always come back negative, blood tests too, she only found out thought ultrasounds - but I have no signs - I haven't been stressed, haven't been on the pill, I went and got tested for std and I don't have any, that was before this - and I haven't been with any other guys - so I don't know what's going on??

Your periods depend on your biological clock and on your hormone levels.
If you traveled to different time zone your body's clock can be confused.
If you changed your diet significantly, if you started exercising, your hormone levels could change.