Cramping triceps!

ThIs is the weirdest thing but after I finish my tricep sets, later on in the day, they cramp up if I extend them back. Like if I stretch back, they'll start to tighten up like a cramp would in your calf. I stretch before and after working out. I use two different sets of light weights, a 3 lb and a 5 lb. I do a bunch of different exercises, not the same ones over and over. I work out 3 times a week. Any ideas of how to stop this?