Regarding the feel of having more weight in your midsection....
How well my pants fit at the waist can be VERY different from one day where I'm holding a lot of water to the next where I could be drained out. On the "bloated" day, pants might feel like they're practically cutting into me when I put them on, they're that tight, but the next day, they might feel loose, so water weight can make a big difference as one factor. This can also be greatly affected by factors such as taking creatine (which tends to increase bloat), or, by eating a lot of carbs and higher sodium foods that might add to it as well. To test bloat factor, it's pretty easy for how I've done it - when I'm compelled to "drain out", I just make sure to have 2-3 days of lower carb eating (usually about 100-120g max, all low glycemic carbs for what I do eat), tons of protein, discontinue any creatine and avoid excess sodium, and I can get rid of any excess water weight in a short time. There are faster ways to do it with things like mild diuretics like dandelion root extract, but I prefer to just get to that drained state via diet.
Another factor to consider is that, if you are currently doing more compound lifts like squats, deadlifts, overhead pressing, etc., all of those things will add a bit of mass to your core as well from the stabilizing factors. I don't train abs at all, haven't done it regularly in years, but my abs are big as hell from the years of heavy compound lifting. The more you do compound lifts, the more your body will accommodate by helping bring up some of the other muscle groups that are also used in core stability, which can increase your midsection size a bit and make it seem like you've gained fat, but some of it may well just be new muscle that's been put on that you didn't even think about.
Could be one factor, could be both, but I wouldn't worry about it too much if you've been eating a pretty clean diet overall and aren't just stuffing yourself with everything you can eat just because it's there. Chances are, with a slight change-up in diet, you'll probably be pretty much feeling normal soon enough.
"A 'hardgainer' is merely someone who hasn't bothered to try enough different training methods to learn what is actually right for their own damned body." - anonymous