Totally normal, don't worry
My DD had one appear shortly after birth on her right shoulder blade. The Pedi noticed it, and told us it will get bigger, darker red, and may become raised- its all totally normal. As you have read, they usually go away as your baby grows. DDs was blood red and you could feel (and if she was on her stomach- even see) it through her clothing. It started to flatten out around age 1 and has faded quite a bit as well. No big deal, and just keep in mind- it'll probably get 'worse' (darker, larger, and more poofy) before it'll get better, but its no big deal.
My pedi said these are totally harmless, depending on where they are on the body. Example: My sister had one on the corner of her eye, and as it swelled, it was causing her eye to droop and started messing with her vision. Since hers was becoming an issue because of where it was on her body, we had to give her medicine- it went away and is totally unnoticeable now, and hers was HUGE...