This is totally normal---but freaks you out just the same because what if it stops, right!?! The first 3 months are normally fastest. But the closer you are to your goal weight, the slower it gets. Each surgery has a bit different loss rate, but if you stay the course, even as you slow down, you will get to goal. If you have hit 50% of your excess weight by 6 months you are doing really well according to most surgeons.
I definitely didn't lose 50% in the first 3 months, most people I see lose 50% by month 5 or 6. But it also depends on how much you have to lose and your individual body's pace. Some lose it really rapidly and are at goal by month 7 or 8. Some take 18 months to hit the same goal. My body is a slow poke and likes to have 6 week stalls! So I hit my 50% mark at close to 5 months but have been stalled for a month since and actually regained 6 lbs, lost it, and then just gained 15 lbs in fluid from surgery again. My weight tracking app looks like an absolute mess. LOL
Working out can seem to cause stalls and slowing down too, because you are trading fat for muscle and muscle weighs more. Plus the act of working out changes the fluid balance and such in your body, so sometimes you appear to gain some weight when you start working out in earnest, but it'll balance out over time.