Sorry you have joined us but welcome nevertheless! I can sympathise with the soldiering on attitude. Many of us have been healthy and very active before diagnosis. Many of us are still healthy and active! And cancer is not so easily found as you might imagine - dense breasts, lumpy breasts, many things can conspire to disguise. Don't look back, look forward.
Fluid build up is not uncommon after surgery. Your body is producing fluid and nutrients to help you heal (think of the fluid in a blister). Unfortunately the bad bugs we all have in our systems think this is all their Christmases come at once! They settle into the nice warm wet fluid and you can get an infection. Keep an eye on it, you may need antibiotics, you may need fluid drained, if you have a breast care nurse talk to her about it. Come here any time. Best wishes.