When I ended up in the ER with anaphylaxis from a bee sting a few years ago, the doc explained that with allergic reactions to toxins, instead of building up a resistance you typically build up a sensitivity. You finally reach a tipping point where your body overreacts and goes into anaphylaxis. He asked me to recall the previous time I'd been stung, and it was stepping on a bee in our yard. Instead of a little localized swelling and itching, my whole foot and lower leg swelled. Just like your thigh.

So, be aware that you may have reached your tipping point. Might be worth seeing an MD about it and asking for an Epi-Pen prescription.