I am now about a week late getting my period, granted my cycle is never the same every month but it is a week later than last months period .I took two pregnancy tests both negative and I just really don't think I am pregnant. Ok my question is, is I started taking diet pills about 3 weeks ago and about a week ago I took a morning after pill. Could that be affecting my cycle?

The diet pills accompanied with the morning after pill could very well be affecting your cycles. I would suggest you wait another week and test again. Be sure to use first morning urine.