I have an obese client who I train twice per week for 30 min. sessions. Twice she has expressed to me that after our workouts she is so damn tired the rest of the day. We are only doing 30 mins and it consists of beginner exercises such as BW squat, res. band presses, & TRX rows. I review her food log weekly and her diet is very good, as well as her post-workout meal. I think a good part of it has to do with sleep issues.
Each obese client is different with regard to their own threshhold for exercise. Assuming all is well with your client medically, it could just be the form and intensity of the exercise combined with her own bodyweight. Because of their extra weight, sometimes even small amounts of exertion can be overwhelming to their respiratory system and joints. If she is new to exercising, this can be a huge demand on her system. How is your client during the sessions? Is she too winded or able to talk to you? Is she well hydrated? Is the strength training “too much too soon”? How much sleep is she getting? Just some questions to consider while you are training.
I would go slow with her and just monitor her movement patterns and exertion levels.
Perhaps you could lessen the intensity of the exercises (move her feet during TRX to lessen the amount of bodyweight or use a stability ball for shallow wall squats to reduce the amount of bodyweight) for a while. Also, maybe just focus on adding some low intensity cardio with her. Just focusing on movement of the body while walking and monitoring her level of exertion may help.
I hope this helps. If it doesn’t get better, I would refer her to a doctor for further evaluation.