I also don't think progesterone changes your cycle at all. I am wondering if possibly the IUI delayed your period? Were there any fertility drugs to take before you had it? I know I am on chlomid and have been told my cycle could lengthen as a result. Or as Rachel suggested, stress could have been the culprit. It doesn't sound like a miscarriage to me either, but anything is possible.
Take care and best of luck to you.
It does not sound like a miscarriage. Sometimes cycle can be longer due to stress and I would imagine that you are under a great deal of stress, as ttc does that!!! I am not to sure about the progesterone tabs, because after I had a m/c they gave me Prometrium (progesterone pills) to start up a new cycle. I know that if you are pregnant and you are taking progesterone that you have to continue taking it until the end of the first trimester, but I am not totally sure that it would delay your cycle. I could be totally wrong.
Sorry if this is of no help. I hope you get your BFP real soon!!!
Rachel