Workout dvds/gym

Hi all been on here for a few years I was once 15 stone went down to 9 and a half stone using myfitnesspal but then started a new job had to give up the gym due to working hours and have gained a stone and a half back in 2 years. I am back again to try and loose the weight I've gained, trying to get get to the gym again as much as possible but maybe only making it 2 or 3 times a week. So have found out all my jillian michaels dvds I brought years ago and never used.
My question is at the gym I can row for an hour on a concept 2 rower 28 strokes per min and above consistently. Treadmill I can walk for an hour on rolling hills on speed of 6.2 and do 30 to 40 mins on crosstrainer (I only do 90 mins per gym session a mixture these are just examples) yet when it comes to say shred it with weights after completing that I feel like I am dying and going to throw up, everything is aching. 30 day shred the same there are some things I can't do push ups that kind of thing. I have all Jillians dvds I thought I was at a reasonable fitness level I guess my question is why am I finding these dvds so so tough and am I not infact at a very good fitness level in regards to duration I can go at the gym?

Any tips and advise woyld be greatly appreciated. I have gone up 3 dress sizes and just want to loose as much of the weight I've put on as I can and tone up.