I can tell you the same thing happened to me. I seriously thought I had damaged my implant. I was so concerned my doctor finally had me make a special trip (I drive 2 1/2 hrs one way) because he was afraid with my complaints I had a stress fracture and wanted to x-ray it. Everything was fine. I'm telling you it just is too much weight and it is too soon to be doing weights. My doctor is known for accelerated rehab and he didn't even have me try that until after 3 months, and as I said before I felt that was too soon. 4 months is probably a more reasonable time frame. As far as knowing how you will feel in a couple of months, your new knee will continue to improve if you will quit abusing it and allow it to heal with progressive exercises and strengthening. By 6 months you will feel almost normal, at least I did. But just because you can leg press 75 pounds once in PT doesn't mean you can do it at the gym with the same results. For one thing, the equipment at the gym may not be the same. What is 75 pounds on one machine may actually be a little more on another. And you don't always feel the after effects for several hours, so listen to your body and knee. Seriously, stop ALL activity and just ice and rest for a few days and allow your knee to recover. You will be surprised at how much better it will feel.