Hi
Warm welcome to the club no one wants to join :-/
Sorry to read about your symptoms worries. You are right to follow your instincts and get this checked out. Anything that feels different or not right usually means something is wrong, after all you know your own body best. You need answers if only to rule cancer out and ease your mind.
Chances are this is something benign as around 80% of referrals to a breast clinic are found to caused by something benign but even benign conditions can either grow or become worse over time and need treating in some way.
You are correct there isn't always a lump that can be felt, could be quite small and hidden deeper in denser breast tissue, some forms of breast cancer doesn't form a lump as such, just a scattering of cells. Best way to self examine is with soapy hands.
Do mention to the GP you also have underarm pain as well just to try and convince them you need a breast clinic referral, otherwise you may get fobbed off 
The only sure fire way of getting answers is with either a mammogram and/or an ultrasound scan. If anything suspicious is then spotted small biopsies could be taken using a hollow needle.
Hope this is of some help at a worrying time, fingers tightly crossed this turns out to be nothing of concern.
Hugs, G n' J
