I would just call the doctor and see what she thinks, but I also think it's probably nothing. My sis said in her 3rd trimester her baby slowed down a lot, and if it happens to me not to worry. I think they might just run out of room. I've heard to drink a glass of juice to see if that makes her move a bit more (the sugar).
My baby has been very active the last couple of days and then, today, I've noticed very little movement (but DH is away on business and I've been chasing after our 3 year old son...so I've hardly had a moment to just sit back and pay attention!) It's normal what you're noticing, I think. As the baby grows, you'll notice the movements change as well - the bigger the baby gets, the less space there is to move around.
I didn't even know you could purchase a fetal doppler - interesting! Kind of reminds me a bit of Tom Cruise purchasing an ultrasound machine..... ;-)
Try not to worry. My babe is incredibly active most of the time and then will have a big rest for a day or 2. I had an appt with my Ob late last week and he said that the baby has now turned to settle head down (I'm 27 weeks as of Sunday) so he suggested that the quiet-ness was the turning and settling time. He also said they will rest during rapid growth just like when they are out and about and they feed endlessly and seem to stay awake for days and then seem to be starving themselves and sleeping all the time.
For the peace of mind without the constant doctor's visits, have you purchased a fetal heart dopppler? That way you can check on the heart beat whenever you feel like it. I haven't got one but a friend of mine has and she uses is quite regularly just to check that everything is OK. She says it's great! I'm not sure how much they are, but she got hers off e-bay for less than $20. Just a thought. All the best!!