Wondering if my lab puppy has incontinence or just misbehaving?

My 8 month old lab puppy has now peed on my bed three times. The first time was roughly when she was about 4 months old and I was watching a friends dog and I thought she must be jealous and was punishing me. Then last week she did it again but that night we cut her nails and she hates that. Now today she was laying next to me and I noticed it was wet and when she stood up there was yet another spot and today I had to throw one of her toys away cause it was broken open. So I am wondering cause she doesn't do this anywhere else but in our bed if she is misbehaving and trying to tell us she is upset with us or if she truly may have a problem. She isn't drinking lots of liquid or acting like she has a bladder infection it just seems like maybe it isn't a medical problem but more of a misbehaving puppy problem. I would love to have any advice and if is is a misbehaving problem what can we do to prevent this, cause now I am gonna have to go out and buy an ew mattress? Help please!!