D.B.
Motherhood is challenging, and those little newborns just don't come with an instruction manual.
Here are a few guidelines:
1) Most things are not emergencies.
2) Some things are. So you have to learn to tell the difference.
3) Your pediatrician has a 24/7 phone line. After hours, try to call if things are really important, but during regular hours, you can call for small things when you just need reassurance or guidance. The doctor or the nurse will tell you what to do, which might be "nothing," or which might be "come in and let us look." Very rarely will it ever be "Go to the emergency room" but you can trust that they will advise you if it's indicated.
4) Learn to keep a list - on the fridge, in your phone, anywhere you can consistently list things. That's a working list to have handy when you call or when you have an office visit. Include things about changes in eating, peeing/pooping, temperature, behavior. Sometimes you'll cross things off the list because they go away, but sometimes it's a really helpful timeline when sleep-deprived parents try to reconstruct when something started up.
5) Take a pediatric first aid course.It's so helpful and so empowering to learn how to distinguish emergencies from non-emergencies.
6) Join a mom's group if you can. Talk to your library (or check the bulletin board), your town's office for children (if you have one), your local Y or JCC, groups like the Newcomers Club (ours doesn't require people to be brand new residents), or groups through church/synagogue/mosque. Getting together with other parents really helps you gain experience and confidence.
7) Make friends with your librarian and find some great books on parenting.
8) Don't be afraid to ask for a second opinion or even change doctors if you really feel you aren't getting answers.
So, regarding this vein - see point #3 above. Call the doctor, express your concerns, give an estimate of how long it's been there, and ask for guidance. My guess is, they'll tell you it's okay and they'll check it at the next well-baby visit, but ask anyway.