I would atribute it to lack of space to move. They are pretty cramped in there but if you are feeling movement thats great. You can ask your dr at next appt how many times you should be feeling movement at this stage. Are you sitting and relaxing and concentrating on feeling baby move?? Thats what I do is lay on the couch or take a bath and really think about how much she is moving and not about anything else.
I'm due May 9th too and I have that same thing happening to me. Some days she will wiggle til no end, but most of the time she is stubborn and will only move a few times. I was told by my doctor that it's nothing to worry about because the baby is probably dropped inside us by now and babies usually don't move as much when they are in that position. Plus they don't have as much room to move like they did a few weeks ago. Our babies are now a little over 18 inches so I imagine it's pretty cramped in there. However, if you are still really worried call your doctor and let them know so they can check you out :)
Baby probably hasn't got much room too move about
As much as before. Their movements do slow
Down towards the end...
If your worried call your mw or go and see her.