Is it just the one, or are there many? If there are more, I would recommend being examined by a physician, and getting the lump(s) tested for HSV tomorrow!!
If it's just the one, I would still recommend getting tested, only with a blood test instead--an IgG HSV type specific test. Since you're sexually active, I would get tested ASAP, and try to not have sex until you know. If you cannot wait, at lesat protect your partner(s) through condom use.
Oh sorry, i forgot to add - it's a very hard lump.