Were you DUE to start your period yesterday? As said above, you could be pregnant or it could just be due to stress, illness, medications, changes in your hormone levels, etc. If you were due to start your period yesterday and you had sex a couple weeks ago, I would do a pregnancy test to start with. If you aren't due to start your period until a week or so from now, it is possible that you are pregnant and what you experienced was due to the baby implanting in your uterus. If this is the case, the urine tests would probably still be negative for a couple weeks.
is it possiable you could be pregant if so I would go to docter
it might just be stress. sometimes also your period changes a little and i think thats normal. if you google it tho itll say something about being pregnant...lol.. but i think if you google anything about stomach, sleepiness, or female parts/problems itll tell you your pregnant. lol