I know you mentioned that you take pictures when you weigh yourself so don’t think your gaining muscle but I find this very unlikely, especially given you lift weights. Any exercise will cause you to burn fat and replace the fat with muscle over time. Muscle is heavier than fat, hence why very fit athletes often weigh much more than the average person of the same height who is less fit. This would also explain why you were losing weight at the start (you were burning fat) but then started to lose less weight (while you’re still burning fat, the fat is being replaced by muscle).
In terms of your 1200 calories per day, this is quite unhealthy - you need roughly 2000-2500 calories per day to provide enough energy for your body to function properly. Maybe instead of limiting calories focus more on eating better (it may sound like the same thing but I’ll explain more). If you’re limiting your calories you may be having 1 proper meal per day and snacking through the rest of the day or something similar. Eating healthily would mean having a healthy breakfast, lunch and dinner and maybe occasional (2-3 per day) snacks per day that are also healthy. By doing this you will be providing your body with healthy food, which will give you more energy. Also, calorie deficits can be the opposite of beneficial when you’re trying to lose weight because it can reach a point where your body goes into ‘starvation mode’ and then causes you to gain weight that you wouldn’t gain by eating healthily because your body basically starts to worry that food will disappear and so makes plans for if that happens, including transforming most of the food you eat into fat so that, if needed, it can get energy from the fat if it stops getting food.
I understand that you’re trying to lose weight so are probably very focused on the number on the scales. I saw something about a specific type of scales that doesn’t only measure your weight, but also gives your body fat % and muscle mass and things like that which are more important than weight (going back to the fit athletes weighing more than people who you would expect to be much heavier than them). You might want to have a look into this as it may help you understand how your body is responding to you trying to lose weight and you may be reassured by seeing the body fat % going down, even if your overall weight isn’t going down as fast as you may like.
Another thing I’ll mention is that sometimes you can’t really see any difference between photos that are taken quite close together. If you compare your most recent photo to one taken 5/6 weeks ago you may be able to see a much more noticeable difference.
I don’t know if this has really answered your questions but I hope it explains what may be happening and why your weight loss isn’t linear