Based upon your LMP, you would have been around 5 wks and 5 days pregnant. Which generally around 6 wks one can start to see a heartbeat. So being around 5wks there would at least be a sac etc. If nothing was found at all, then chances are you're not pregnant.
If you are not pregnant, and you are not having any bleeding or cramping, then chances are there is no loss either. I don't know if they checked your ovaries, as an ectopic pregnancy could be the situation, or it could be hormone related as well.
Getting a blood test to check your hCG levels is most important at this point. If you do start to feel any lower right or left cramping, do be seen right away. Just to make sure it's not an ectopic pregnancy.
There are several causes for irregular periods. Examples; stress, weight gain/loss, medication, illness, climate, cyst, or polyp of the cervix, hormonal imbalance, thyroid, infection.. just to name a few.
So do keep your appt in a few days, but also ask your doctor to run a blood test as well.