Horse Lame on Hind Left

My horse is a 5 year old Paint mare (mostly Thoroughbred). She is 15.2 HH, and pretty athletically built.

She was in good condition coming out of winter. During winter I let my horses have a little more 'down time', but not getting out of shape. Anyhow, after winter she was doing great. I started jumping with her again, and she was doing fantastic. Then about 3 weeks ago I gave her 2 days off due to bad weather. When I went to lunge her for her next workout, she was lame, and sore to the touch on her hind left. I hosed it off daily and gave her time off. In between occasionally I would try to lunge her again, but she was always lame.

Today I had her out again. At first she looked better at the walk, but definitely sore at the trot, and it only got more and more pronounced. Her quad and hamstring area is swollen, a little warm and slightly sore to the touch. When she walks, its like she kind of drags her back leg with, and if she does lift it up, it kind of goes out to the side and up.

Has anyone had anything like this before? I'm worried about her. After this much time off a normal pulled muscle should be healed (I've had horses with that in the past, and that's why I've waited till now).

She is out on pasture, so she can walk around all day. She does not have a pasture mate, so she does not run around often. Lately she has gained some weight, she has always been a hard keeper, but now she's on the brink of having too much weight.

Any advise would be great. I can try and get some pictures of it tomorrow. Thanks!

Well, you might want to have the vet out. She may need an ultrasound if he thinks it's soft tissue damage.
I'd coldhose it at least twice a day. If she isn't on it already, I'd start her on MSM. It takes a week to start working. It's an anti-inflammatory supplement that works well in a lot of horses, and it will calm down excessive inflammation in her injury. Inflammation sometimes interferes with healing.
You might also try bute. Yeah, lots of people are against it, but I see no reason not to use it for injuries. It is a good pain reliever, and helps with inflammation too.
I'd be worried it it becomes worse or doesn't get better after a few days. The vet ought to come out to check on it.
Also, don't lunge her. It puts excessive pressure on the legs and can actually make injuries worse. Hand walking is best. Have a friend do it if you want to watch the horse.

Does it ever lock up or crack? Any possibility of an abscess in that area? Agree w/ rascalboy Have a vet look it over.