Hmm, yeh, could be teat. My dd was on slow flow, I bought her medium flow and sometimes she'll do the same. She'll get frustrated and choke and push it out of her mouth with her tongue. I changed it back to slow flow and she'd be ok. But the problem is I think she swallows lots of air from trying so hard to suck lol. In the mornings I dont know why she could be doing it, but at nite I would say they are tired, irritable etc after the day. Also, during all the struggling might be swallowing air, so its worth trying to burp her, even if it seems she didnt drink anything yet.