I think my dog is love sick

My two Yorkie boys have a girlfriend (Shih tzu) a few doors up which I dog sit two full days per week. They don't see her every day but I do ccasionaly bring her down our house for an hour a couple of days per week so they can all play chase or catchy kiss as I have a huge garden.

They havent seen her for 4 days and Marley spent most of yesterday afternoon just whinging, running out in garden looking up the street and barking/crying then running back in and crying. I thought he must be missing Poppy, so I asked him where Poppy was and he got so excited.

Well today Poppy passed our fence on her walk. My two would usually bark and chase dogs around the fence but this time it was an excited bark mixed with a cry. I then noticed they ran past the front window so I knew they had somehow manged to jump the side fence (which separates the back and front garden) - something they have never done. They both ran straight to Poppy all excited. I put them back in the garden and they done the same again (little monkeys can jump so high) so I have spent the aftenoon nailing planks of wood to heighten the fence.

Marley has whinged on again for most of the aftenoon and soon as I ask if it's Poppy he's missing he gets all excited. I think he is love sick! It's funny because when I have Poppy here she really picks on him, she is trying to be boss I think. She jumps up at him, pulls his fur and manages to pin him down play fighting - he is so soft so doesnt dare move then when she chases after him he seems terrified yet he is missing her. It is only Charlie that loves the chase, Marley just kind of tags along barking and soon as Poppy chases him, he runs. They are so funny to watch.

I'm just wondering if maybe she is coming in to heat and these can sense it although they don't hump her, it is always the other way around which I think is odd (possibly domineering behaviour). She is 6 months old which is around the age they have their first heat. Or maybe he has just fallen in love and missing his girlfriend too much