Any one know how to quickly put on weight?

hey all again.

So I“ve been noticing lately that I feel thinner than normal.
I’ve always been skinny but lately I feel to skinny.
I’m 51 kg, height 1.60 so 5’5 I think.
I have a little frame and my dad was never able to put on weight.
However I just lose it so easily and now I’m starting to think something is wrong with me like cancer.
I havn’t lost anymore weight, I seem to be quite stable where I am, but I feel thin.
I’ve tried and tried to put on weight, i’m 33 and want children and know that weight helps you conceive. (as well as sex)
so… does anyone have any good ways to just get fat!

Thanks