Hi there ,
Most common reasons for this are .
Mis calculating what you ate ,ie a few small snacks here and there that you didn't think were much but didn't add in
Over calculating your calorie requirement , for many of us who aren't very active in our normal day dont actually require or burn the calories the calculators tell us we will .
Work outs are great but depending on the style of workout we can again over calculate calories burnt as a result .
Lastly medications and hormones can wreck havoc with our best efforts to lose fat. Birth control thyroid meds anti depression meds and many more things we are given by doctors these days can often cause us to stall our weight loss or even worse put weight on .
Lastly make sure your weigh in wasn't just a bad day for bloat or carrying water . Some weeks you may not move at all or even go up but then the following week you drop more than expected .
Just a few ideas from somebody who has no idea of your situation