What Causes A Tender Yellow Blister On The Chest?

Yesterday felt a sudden pinching pain on my skin when I moved my right arm across my chest. Thought an ant might be biting me or a sliver of wood was caught on the edge of tank top sleeve and poking my skin, but when I pulled aside my tunic and tank top sleeve and searched my clothing and skin I couldn t find anything other than a small reddened area on the skin overlying the pectoralis. The red spot quickly swelled and formed a yellowish blister. It s not very large, but is still irritating and tender, and there is a lump in my armpit, which I cannot say was or wasn t there prior yesterday s incident . I was raking mulch into the top of a raised bed when this occurred.