Fish is pregnant guppy & only using 1 side fin...possible fin rot??

[Nature Momma]

Okay, so I bought these fish 5 days ago to raise as feeders for my slider turtle. I got lucky & was given a pregnant one that's about to go any day now... but she's acting weird. I've tested the water w/all the necessary tests, water doing great! No other fish seem to be having this problem. She is eating. I'm not sure how far off she is from giving birth. She swims near the top of the water but doesn't look to be gasping or suffering from lack of o2. She sort of "bobbles" when she's trying to swim in place, or maybe sways from side to side. Okay, so I've tested the water w/all the necessary tests, water doing great! No other fish seem to be having this problem. The fish is eating. She is pregnant but I'm not sure how far off she is. She swims near the top of the water but doesn't look 2 be gasping or suffering from lack of o2. She sort of "bobbles" when she's trying to swim in place, or maybe sways from side to side. If looking at her, (her looking at me) her right fin is sticking strait out but not moving at all. Her tail fins have what appear to be 2 black looking places, specs, or spots. 1 in the center of the back fin & 1 at the bottom of the back fin(as seen in 1St photo) 2nd photo is of her side fin sticking out motionless. I've also noticed at times she will get close to the filter & swim against the current as if she's trying to remain active/busy. The only time she goes to the bottom is when she's going under the log to sleep with the light off. Someone please help me! Am I doing something wrong? Should I separate her from tub heI want to keep her until she has her babies at least. But I'm stumped as to what's going on w/her. If u need better photos, just let me know! I'll be glad to answer any questions so feel free to ask. Thanks in advance!