Hi The lump sounds like a lymph node that's irritated, l have had one that sat there for years my breast surgeon said it was ok, to leave it. It became inflamed and I had it removed about 7 years ago. It was not cancerous more of an inconvenience as the wound leaked for a while before it healed 
l would see my doctor to get the lump checked. If it doesn't go away. 
take care 

Susie 

Hello and thanks for your post,

I am sorry to hear about your symptoms that you are experiencing. I would make an appointment with your GP so they can examine the area under your arm. There maybe a simple reason to explain your symptoms but we can't diagnose you.

Swollen lymph glands do go up and down and usually are a sign that the body is fighting an infection but it is always a good idea when symptoms don't seem to be settling down to go and get them looked at.

The NHS have produced some information on swollen lymph glands which you may find helpful to look at here

When you have seen the GP and your pain has been assessed then it may help to do some gentle exercises but do ask them first.

You may find it helpful to look at the information about shoulder pain written by the NHS here

and some more exercise ideas here 

I hope some of these links may be helpful and you manage to get the pain and lump assessed.

All the best,

Catherine