Health Concern

TW for family death just in case you're sensitive to that I'm gonna start off apologizing if this is seeking assurance, I'm trying my best not to do that but it might turn out that way because this is really involved with specific intrusive thoughts I have and I'm wondering what to do to cope with this in a hopefully healthy way. About 1-2 years ago I noticed under my left arm I have a noticably larger bump there than under my right arm (I know that it's normal to have a bump at all in that area is normal because of having boobs and many people with boobs do have a bump under their armpit) and I went to the doctor about it among other things. They said it was probably a lymph node fighting an Infection and to come back in 2 weeks if it hasn't changed or gotten any bigger. I was worried about it for about that amount of time but when I brought it up to my dad that it hadn't gone away he said what they said so it might not be anything. Over the time since then I still haven't been to the doctor to have that checked on and I've been getting on and off intrusive thoughts about the bump. There are bouts where I just can't stop being aware of it and checking on it every time I'm in the shower and it's bothered me so much. In October, my grandpa died of cancer unfortunately, and I think that had sparked these worries to become more intense. For the past 2 or 3 months, most of the time when I'm showering or even while wearing clothes, I'm incredibly aware of it and constantly checking the size of it or if it grew. I've had panic attacks in the shower at least a couple times because I'm so aware of it and I'm automatically worrying that I have breast cancer and that I'm going to die and envisioning that reality. Showering has become really anxiety inducing for me because of this and even not having a shirt on is hard because I'm so worried about this. Mainly what I'm wondering is what I do about this worry. I know that a lot of advice when it comes to intrusive thoughts is to sit with the worry, but does that apply here? Should I try to convince my dad to take me to the doctor about it again (its been over a year or 2 since I went) or would that be playing into the intrusive thoughts? If it's not the right thing to do, then what is? It seems impossible to be able to just cope with the thoughts of it possibly being cancer, I'm incredibly scared of dying.