I Have Been Having Unexplained Bruising On My Right Arm

I have been having unexplained bruising on my right arm (just began)on the inside of my arm. It is very large, and every day has become larger. It began as a red bruise about a couple of inches around. I had hurt my arm several months ago pretty badly--however it seemed to clear up on it's own. Then, a few weeks ago I hurt it again trying to lift 12 packs of soda and I was unable to hold onto the soda and I needed help. I have always been very strong, so I thought nothing of this. Then over the past few days the bruise on my arm has grown larger every day. It is dark purple, and it is tender. What could be causing this? Thank you in advance for your consideration. Nancy Hembree, age 67