At such a young age, I'd have to say poor bladder control or excitement peeing, if she's been checked for a UTI.
My Dobie-girl, Bella, often had accidents at that age -- she's just a baby! Once she learned to pee outside ONLY (around the age of 16 weeks), she would still pee while I was opening the crate to let her out in the morning, when the dog walker came, and when let out after work -- and also when we had company and she got excited, as well as when she was playing. I was concerned why she hadn't outgrown that stage, and my vet said it's a developmental thing, and she'd grow out of it when she had better bladder control. She is 6 months now (well in 2 days she will be, lol) and she hardly does it anymore. Once in a great while, if I don't open the blinds and the locks on the doors fast enough, she will get excited and pee a very small amount, but it's nowhere near as frequent or as much as she used to.
I thought it was just Bella that was a little slow in gaining bladder control when excited, but when I got in touch with the owners of two of her sisters, I found out that they did it too (albeit to a lesser extent than she). I guess some mature and develop faster than others!
In your case, she's too young and immature to be "marking" so I would guess it's more excitement peeing than anything else. I wouldn't yell or "correct" her for doing it, either. You can say simply, "No pee-pees in the house." and leave it at that, since it's really not something she has control of yet, but at the same time, let her know this is not something you agree with, either... Until she is mature enough to hold it, just stock up on LOTS of paper towels and urine cleaning supplies. Nothing else you can do about it!