Can’t lose weight.

I am an 18yo male. I have been doing my diet for 3 weeks now and it all started good. In the first week it was all good I lost 3 kilos, but since then I haven’t lost any. After the first week I started going to the gym I usually wake up at around 11.30 so I dont really have breakfast, for lunch I eat chicken breast with grilled vegetables and a little amount of sweet and sour sauce. No dinner either. I usually drink around 3 liters of water. I also drink Raspberry flavoured Zero water. Please help what I am doing wrong.

I did the same routine last summer without the gym and cardio and there wasn’t a period like this then.

Can’t lose weight.