For the past week or so my 4 month old has been pulling off during nursing sessions, arching his back and then grunting/straining/fussing. At first it was every session, but the past two days it has just been during his bedtime nursing session. Any ideas on what this is? I know there is milk there, because I've squeezed my nipples when he pulls off and there is milk. It's been particularly hard the past two nights because he's gotten so keyed up right before bed. ???