Did your trainer do some physical assessments to determine that the weight gain is muscle??  Are you doing that much weight training and do you see or feel "frank muscle development?"  Has your trainer looked at your journals and approved your menu plans in terms of calories, amount of protein, carb choices and portions sizes and daily frequency?  Are you checking your pulse several times during the workout to see if you are truly hitting aerobic rates of calorie burning consistently??

So many of us just make assumptions on all the above and the "fragile balance" of creating calorie deficit or burning calories with exercise really has to be objectively determined and observed.  In plain lingo - that means that most people innocently think their journal is clean -and sometimes it isn't, or they are eating bigger portions than they realize or they think they are burning alot more calories than they are - it is truly human nature.  So the best way to get to the root of the problem  - is to possibly involve your trainer more AND think about a consult with a nutritionist or dietician.

I've been helping people for more than 20 years and by and large it is troubleshooting what they are doing -really dissecting it - that often helps to shift the situation.

I do tell clients to do different aerobic exercise daily so your body constantly remains challenged - I run 2 days a week, use the elliptical once a week, play tennis for 2 hrs once a week, use the stairmaster once a week - so I really have it "hard" everyday. My daily aerobic exercise is 50-60 minutes plus 2 days a week I do 30-40 minutes of weight training.

I also tell clients to measure every portion of food for 7 days and take note of total daily calories  -  so we can really analyze.  that's what a nutritionist can help you with.

Hope some of those suggestions help.