Worried baby maybe smaller? Help

Ok so im meds for adhd been on it for 7 years then a suprise pregnacy and cant seem to come of the meds so dr has me on lowest dose she said the only thing is baby could be smaller but thank god she was acually bigger on ultra sound so this morning my 33 week apt she said im messuring few weeks behind now and next apt she wants a ultra sound to check fluild and make sure she is getting bigger im dialed 1cm my this is my fourth child 37 weeks she thinks i will deliver at just like my last pregnacys she says babys so low that maybe thats y i messurged behind anybody have this happen im worried