What are some reasons for why this is happening?

My period is late about 4 days and I'm not sure what to do. I have had my period for a really long time (about 10 years!) so its not like I just got it and my body is still getting used to it. My cycles are usually pretty regular but once in 2 years may change. The last time my period changed was last August and I ended up getting it 2 weeks early. It had been arriving steadily ever since. Usually before my period I would get cramps and about a day before the cramps would be really bad. Now I don't really have any cramps and if I do, they are only slight causing a slight uncomfortable feeling and then they go away. Could I be pregnant? I have not been stressed and I am not on any medication. However ever since school ended I have been sleeping rather irregularly. I go to bed really late and wake up late but in total get an average of about 5-6 hours of sleep a night. I also feel really moody and irritated. My weight has not changed (gain/loss) so I don't think it is because of that. I also had sore thighs today and felt very dizzy (saw spots) for about 5 minutes. Please help. Thank you.