well, this can be complicated to be honest with you. a gain of 1.5lbs/wk doesnt seem like a lot, but at the rate that people build muscle, logic determines that you arent gaining 1.5lbs of muscle per week. water, glycogen, etc, etc...at 1.5/wk yes you are gaining some fat. if your body composition is staying the same as you gain weight and get stronger, that may be acceptable to you if your goals are mainly or purely powerlifting. if that's the case, keep eating as you are quite light for your height by PL standards.
if you dont personally like the way you are gaining, make adjustments NOW. There is no reason, after gaining nearly 20lbs, if you dont like the composition of that 20lbs, to keep going and expect things to change for the better moving forward. in fact, in my experience (certainly backed up by many articles here as well) the more you gain, the more those muscle gains diminish. you may be gaining 3:1 muscle to fat right now, and that may gradually shift to 2:1, 1:1, etc. I am not trying to freak you out, or tell you to lose weight or even stop gaining weight. But be honest with yourself and your goals, maybe it's just a matter of cleaning things up moving forward. The best way to gain and not turn into a fatass, is to KNOW what you are eating. "around" X calories will not cut it. It's very easy to underestimate when you arent keeping meticulous track of things.