Small lump on face - should I be worried?

[chachabing]

My mare has developed a small lump on the left-hand side of her face where the noseband sits. I noticed it about two weeks ago but assumed it had been a tiny cut which had scabbed over, a few days a go I noticed it had grown and it has began to really worry me. It's small about 3-4mm in diameter and is raised about 3mm; its smooth at the sides kind of bumpy on top and is dark in colour. As soon as I noticed it a couple of weeks ago I removed her noseband from her bridle but obviously the headcollar sits in the same place and does put against it, it has bled a tiny bit a few times where tiny cracks in it have appeared and the top layer of skin has come off. I've had a good feel of it and it appears to be deep, if you get what I mean? It's not just attached to the skin? So it's definitely not a fly bite or a cut and although I've got no direct experience of sarcoids it doesn't really look like one and horse has never had them before? It kind of just looks like a wart? Do horses get warts? Or are all warts and cancerous type lumps under the sarcoid umbrella?

Any ideas? I know many of you will say to call the vet, just sure how long I should give it? Should I give it a week and if no improvement call, or longer or just get him out right away for peace of mind.
Will feel silly if I call out and it drops off in a few days.
Thanks in advance!