I have these symptoms when I get very bad gas. For me it's because I'm lactose intolerant (and didn't take enough Lactaid when eating dairy). It's sort of convenient to be able to rub your left breast to burp yourself, but it's also very sore and uncomfortable the whole time so it's not fun. Usually the soreness/burping comes a few hours before farts start releasing gas as well.

In short: I'd bet it's gas, and you should figure out why you're getting gas (lactose intolerance, IBS, etc). I have no clue why it hurts by your armpit but it does... this has been very repeatable for me.