Unexplained weight loss?

So a little back story I'm 12 months postpartum

My pregnancy weight was 149 and I went to 210 by the time my son was born and in the last 5 months I've dropped down to 142 and it's still dropping and I have not changed my diet I'm starting to get worry cuz I can barley fit in a size 7 jeans I haven't been in a size 7 jeans since I was 15 I'm starting to worry because it's been less then five month and I've dropped a good amount of weight with doing little to nothing ..

Should I be worried or is this normal ?