swollen and sore arm pit!!

As you can see from title have hurt back so been off work for 5 weeks now.. having had back improvement spoke to gp to try and be responsible to prevent further sickness from work by having flu jab! Had swine flu last christmas!!
Anyway gp said great idea and I had it for first time ever on tues morning.

Anyway yesterday Wed started to feel sore under right arm.. had jab in right arm. Just kept feeling sore when rubbed on my body. Thought it may be swollen but thought nothing more anyway this morning under armpit is swollen.... kind of large area, fluidy not like a lump or anything. Rang docs and they said come in. GP looked and said when prodded deep ( bloody hurt!!)could feel something, maybe ligament or lymph but deep inside. Just said urgent ultra sound scan!!

Now Im scared!! Any ideas, I did say had flu jab and could that be related?? She said could be cyst or lymph fighting an infection?? Had blood test last week, before flu jab and platelets were 435 which is just over normal.. so she thinks I could have some kind of infection going on?!!

Just confused and worried now, told to go home rest and take paracetamol and ibuprofen!!
It remains sore and swollen. Of note had breast lump checked in June so had all mammograms and ultrasounds then including under arms!


Any ideas, experiences or advice....