I wouldn't worry about that... your first two pregnancies may have been felt earlier, but every pregnancy can be so different even with the same woman. i've met women that didnt feel the kicks until after 20 weeks. i've heard people say you feel them easier if you are skinnier, but i dont believe that entirely. i am very skinny and i did feel my baby's first kick at about 15 and a half weeks... 3 very strong and distinguishable kicks was the very first thing i could feel from her (i may have felt quickening before that but i am not sure, i could not say if it was or not) but i knew girls who would swear they felt quickening as early as 10 or 11 weeks, and girls that did not feel anything until 22 or 23 weeks (of all body structures) it is normally said that your 2nd and so forth pregnancies, you feel the quickening more because you know what it feels like already and are looking for it, but that is not always true either. i've come to realize that when my daughter has her back facing my stomache, i dont feel hardly any... if any at all, kicks. if she is facing my stomache i feel kicks galore (noted from various ultrasounds and exams) also if the placenta is between the baby and your tummy, it cushions the kicks and you may not feel them.
just dont worry :) it's not uncommon for that to happen and it doesnt matter that you had babies before, this one just has his/her own personality :)
on the medications note... sometimes they do make the baby a little more sleepy. its not a bad thing necessarily, just a small side effect. i had a bit of a complication that put me in the er 2 weeks ago and was put on some pretty strong meds that my ob said wouldnt hurt the baby (i've gotta good dr.) but all the while i was on them she kicked a lot less, turns out they just made her more drowsy (not surprisingly they did the same to me... so a little body feeding off mine, it'd make sense for her to loose energy if i do for meds) but she's fine, really healthy, growing fast... and now that i'm done the meds she is back to her extremely (umm.. EXTREMELY) hyper self. lol, im growing a gymnist in my tummy.
so the meds could be making her sleep a bit more, but it's not bad either if she's growing well :)