About Benadryl (and also the anti-histamine doxylamine succinate, both sleeping aids)?

Okay, so i take Benadryl (diphenhydramine) or its sister doxylamine succinate (both anti-histamines, work pretty much the same) to help me go to sleep.. And lately every time I take either one when i am almost asleep i will wake up quickly, catching my breath at least once if not more.. Why does this happen is this normal or is it more serious? And this never happens when i dont take them so i know that that is what is causing it.. Should i stop taking them or is this a normal side effect? It didn't use to do that when i took them a few months ago but now ive stopped for a few weeks and I took it last night and it happened again.. I always only take low doses nowadays (about 12.5-25 mg of Benadry (one pill) maximum) or 6 or 7 mg of doxylamine succinate.. Thanks in advance. (: