Should I be worried? Help!

I'm a FTM and 23weeks pregnant. I've been feeling regular movements from 18weeks and they have been strong enough to see my belly shake recently. However the past few days I feel as though my little girl hasn't been moving as much and when she does, the movement is very light, nothing like what it was before. I've had a bit of a backache but no cramping/spotting etc. Midwife advised me last week not to worry about movement too much yet but it just seems like such a dramatic change, I know she has her lazy days but it's been about 2 days now and I haven't felt a kick like before just light taps here and there, did any one else go through this around 23 weeks and should I contact my midwife again?