Weight loss plateau for 3 months?

I started a diet in the beginning of april and since then I have lost about 45-50 pounds. I was eating about 1800 calories a day and walking for about an hour and a half every other day. I was at about 280 I realized in about late june that I wasn't eating enough so i amped up my calories to between 2000 and 2100 and lost another 10 pounds. Since exactly August 20th, I have lost no more weight. I tried eating more to see if that was the problem and stayed the same and then tried eating less and stayed the same. I also tried to to do calorie shifting and still no change. I have a little problem with exercising because I tore my ACL so I try to do now 20 minutes of kickboxing every other day and walk for an hr on the days i dont. and still NO CHANGE! i keep my calories between 2000 and 2300. Has anyone else had this problem or know a way to break out of this? Im getting extremely frustrated and feel as if I wont lose any more weight ever