Popping sound/feel on left pedal
Hey guys, I'm having a very annoying issue. I have a Diamondback Sorrento bike, for a couple of months. I had a popped front tire a few days ago, so I changed the tube. Ever since then (although I'm not sure it's directly related), when I pedal fully, on the downstroke of my left pedal there is a popping sound that I can feel in my foot. I have no clue what's causing it, but it's insanely annoying.
I tried using some WD-40 on the chain to no avail. I'm not positive, but when I stood up I think it went away, and it's hardly there if I'm going extremely fast.
Does anybody have some things I could try?
I'm not very good with parts or maintence. I usually go to a bike shop when I have problems. I was going to take it there anyway to get my front brakes replace since a pad fell off, should I just let him deal with it? I'm afraid he'll overcharge me for something very simple, though.