Could this be lymphedema or something else?

Hello and thank you in advance to anyone who takes the time to read this - sorry its so long. Two years ago, at age 47, I was diagnosed with IDC, Seven months after finishing treatments, an axillary node on same side as original cancer became enlarged and upon biopsy, it was found to be cancerous, but pathology of this recurrence was triple negative. PET scan and MRI of breast at that time showed no cancer anywhere else. I then had a full axillary node dissection. None of the remaining axillary nodes contained cancer. I then had more chemo 3 months of Carboplatin and Gemzar . All of this treatment was finished about 2-1/2 months ago. Follow-up mammogram at that time showed only calcifications in area of lumpectomy which radiologist indicates appear to be scar tissue. Oncologist says he will follow me closely alternating between MRI and mammograms. Next scans are set for September – which are PET and breast MRI. In the last several weeks, I developed a lot of arm pain and thought I was possibly getting lymphedema, which I've never had before now. I really didn't notice any swelling so I was also thinking I might have just injured my arm – at times lately I've been a little over ambitious with the exercise since I'm finally feeling better after chemo but I did lose a lot of muscle tone after going through treatment for almost two years between my original treatment and then recurrence treatment. Anyway, I have pain with kind of a fullness feeling in the armpit area, a nerve type pain sometimes radiates up into shoulder, and down back of upper arm. I have a 6-month follow up with breast surgeon later this week so planned to ask her about all of this at that time; however, last week when checking for lymphedema swelling, I noticed what I guess I will call a lump, although its not a nodule-type lump, its more of a larger slightly raised area 2 or 3 inches wide and maybe an inch or two in length, just below where the axillary scar is – its not sticking out enough to actually see it when I look in the mirror but when I rub my fingers over the area I can feel what is either swelling or a mass – not sure which. Of course I'm driving myself crazy about what this could be and couldn't get dr. appt. moved up any sooner. If all axillary nodes are gone from that area, it can't be a node so could cancer be growing in the soft tissue there? Or is this just scar tissue from axillary surgery in February … but why am I just noticing it now and in conjunction with this arm pain. Maybe it is lymphedema …. but when I press on lump I don't see any finger imprints so don't think it really feels like fluid. Sigh … one of the hardest realities of my diagnosis plus having had one recurrence already … every pain or problem might equal more cancer?! I would appreciate any thoughts you might have and any enlightening tips you have for how to not spend my life worrying about a recurrence.