both my dogs have red spots on their bellies and under their armpits.?

Are they scratching excessively? They may be allergic to something in your new house or maybe a chemical that you are using. More information please?

One of my parents had a dog that had spots that look like exactly what you described, same size and like blisters filled with blood. They ended up being cancer. It could also be bug bites or an allergy. I would have your vet take a look and figure out what’s going on.

they might have sarcoptic mange. These mites dig into the skin and cause the dogs to itch constantly, resulting in self-mutilation (typically on the belly and front elbow and ears). Not sure if this is what your dogs have, if they look like blood blisters and not scratches. If you think this is what they have after referring to some sites that can help you diagnose, there are plenty of options on how to treat it, not very expensive mostly tedious (similar to when your kids get lice)