Do any of your beagles do this?

Annie is 20 months old. She had her last heat in August. In October she started mounting our cat. Her and the cat are pretty close to the same age. He is a neutered male and they have been best buddies since we brought Annie home, they even play together, and the cat(Andy) will hop sideways and jump on her. In October we noticed, usually when she has just come in from outside she would mount Andy from behind and well..... appears to be humping him. We tell her no and she will stop for a few minutes but soon goes right back to it. This goes on for about a half hour then she gives up and lays down.
At the end of November my sister passed away and we took in her daughters dog, until she could find a place to live that she could keep him. Pee is a 2 year old Neutered Male standard Poodle. Now Annie does the same thing to him she use to do to Andy. I have never encountered this with a female before. Actually my male never use to mount things. Is this a sign of dominance? My husband thinks its because she is close to coming into heat again, but its been going on since October.
Any suggestions? Pee and Andy really don't like it. They both just stand there looking back at her like "WHAT ARE YOU DOING TO ME?" we get after her but as soon as we are not looking she goes right back to it again. Its almost always right after she comes in from outside, and it usually takes a half hour of us telling her No before she finally gives up and finds something else to do.