It could be a fry. If you think she may be having babies, you may want to wait on the water change until after you catch the babies. If you choose to do a water change, siphon the water into a bucket in case you suck out some babies. I've done water changes before while the mom was birthing (didn't know she was birthing till I had already siphoned the water.) and it had no affect on her, but I personally would wait if you can. As per your other post your water parameters are good, so a water change is not essential right this minute.