lump swollen, shoulder on dogue ...advice

my dogue de bordeux xena has had a small lump for quite a while on her shoulder(right at the top of her left leg),
the advice from the vet a few months ago that it was nothing to worry about just fatty lump, so because xena my dog wasnt worrying about it i thought nothing of it,
but yesterday morning after her morning walk she came back home and went as usuall to lie down ,about an hour after i heard her yelp, and when i went to her i noticed the shoulder had swelled with what felt like water, i panicked wondering what it was and phoned the vet and straight away took her,
he said it was an infection and put her on antibiotics, synulox.
she dosent seem to be in pain and it only bothers her when she lies on it shes eating and drinking normally and moving about as usuall but it does worry me , i was wondering if anyone else has had this problem and could advice me why she got it, im thinking of changing my vet as he dosent really explain things to me when i ask questions, so if anyone could help it would be really appreciated, thanks