My dog has suddenly started acting very territorial?

I have a (spayed) female yellow Lab who is 8 or 9 years old and a Ridgeback mix (neutered) male who is about 4 years old. They have always gotten along well, except for the occasional tussle over food (the male has occasionally been food aggressive toward the female, but it's unpredictable). Lately, however, the male has been acting very territorial and aggressive toward the female. When they are playing together in the yard, he will not allow her to come onto the porch while he's on it. They have a plastic pool that they play in, but he won't allow her to get in it while he's in it. Whenever I'm outside, he won't allow her to come near me. He has also started jumping on me every time I go into the yard, which is unusual. I'm not sure what's going on here. Any ideas?