Working out with a trainer and still gaining weight?

This is concerning me a lot lately. I recently started working out with a trainer once a week and its been my 2nd week. The trainer gave me a diet and workout plan to follow. I have been doing everything just how he has told me to. I haven't lost even a single lb. What is going on? I am really confused!
Updates
+1 y
ok so my trainer told me to focus keeping my heart rate in between 117-137. I usually like to do HIIT workouts. These workout are kinda slow cardio with minimal calorie burn for me. So I asked him and he said this will target my fat cells rather than muscles with HIIT. Anyway so the first week I started to workout with him I gained about 5 lbs. Then this week I lost those 5 lbs and since then I haven't lost any. Whats going on?
Working out with a trainer and still gaining weight?
Post Opinion