How can you tell if you were bitten by a spider?

I woke up a few days ago and I had about 3 red bumps on my body that I didn't have before I went to bed. I was outside but I wasn't bitten by anything so I don't think its mosquito bites. The same night i had a dream about a bunch of spiders covering my room and I was terrified, that probably isn't relevant. The bumps are raised and itchy but that's all they are right now. My question is that what happens when a spider bites you and do you think that's what this could be?