Yeah, I think it's from around this age that they can start going fairly long periods between poos.
Still geting plenty of wet nappies? Are teh poos still runny when she does go? If so, she's not constipated.
If she seems really uncomfortable you can try giving her some cool, boiled water (no sugar), but it may not really be necessary. She's probably just getting used to her system doing different things - poor dear.
If you think it might be the medication doing something, could you try stopping it for a day or so and seeing if it makes a difference?