Braxton Hicks or Baby balling up?

How can I tell the damn difference?? I have three kids and have been fortunate to have never felt labor pains due to all my babies being c sections. I don't know what a contraction feels like :-/

Sometimes it happens after I've been on my feet for awhile and I lay down. Sometimes it just happens out of nowhere when I've just been relaxing. I FINALLY have my first scan Wed morning so I'll be sure to have them check me then. But is anyone else experiencing this and how can u tell the difference? I'm 21 weeks.

Thanks ladies!