Bump on 4 month olds leg

So we went to our 4 month check up and my doctor said this might be a spider bite. My sister in law though it was a birth mark.🤷🏼‍♀️ I’m trying to see if anyone knows what this is. I’ve put neosporin on it for the last few weeks and it will form a scab and the scab falls off on its own. I’m most likely going to take her in to a dermatologist if I can’t figure it out. Doesn’t seem to bother her but I just don’t want it to be something serious. Any suggestions as to what it is would be appropriated👍🏻