May a woman wear sweat pants in front of her husband while she is a nida?
From a technical halachic standpoint we can’t say that it is not permitted, because she isn’t uncovered. On the other hand the oposkim say that if his looking at her in this way will cause him hirhur, (it will cause him an inner excitement) then he may not stare at her when she is dressed this way. In general though it is better to be careful about this.
Shach Y:D 195-12, Igros Moshe Y:D 2-75, Shiurei Shevet Halevi 195-7 (2), Chut Shani 195-7 (9).