Hmm, certainly a little weird, hopefully it is just you're body preparing itself and feeling different as you are in a different position. My labour started with period like pain, but quickly progressed to definite contractions!
Like right now im laying in bed having constant period cramps ... its so uncomfortable ... but usually during the day im fine no period type cramps just tightening
I know how you feel, my first two were overdue too and I'm desperately hoping I go earlier this time! Maybe the cramps are more intense when you are lying down with baby's weight being more on one side or the other? I've not had any experience of that, maybe you should call your midwife or doctor?
Fingers crossed for you going early, certainly sounds like you're nearly there, I had braxton hicks regularly for the last week or so before I went in to labour both times!