Im 14 weeks, and yes I too noticed light cramping after we had sex (yay we finally go round to it again - just couldn't face it in the early weeks lol)
but this is like a bad period pain - I was sat indian style (cross legged) last night and sneezed (i seem to sneeze a lot right now) and I thought I was gonna have pushed the poor little dot right out.
I was curled up in pain for maybe 10mins and had to go to the loo to check I wasn't bleeding - I really thought the force was gonna do some damage.
Then calmed down and got real with myself.
I did mention it at my last scan but the sonographer just kinda smiled and said "hmm".
Oh scan - I nearly forgot - I must scan my scan and post it...