Just to clarify from the onset- I do NOT have an eating disorder of any kind.

I eat just a little less than my bf, which I consider acceptable. I eat until I'm full, and often this also includes 'junk' food. Like McDonald's, crisps, fatty meat... And then I eat my fair share of veggies too.

Well, I'm 25, about 1.67m (5'7) and now weight about 50.5kg. I believe 52 is acceptable, so this would classify me as underweight.
I don't exercise more than ride my horse twice a week. I'm not a sickly person either, so I don't consider myself to have a weak immune system.
I've pretty much lost 5kg in 5 months. I don't know what could be going on, or even where to start looking for a cause