Sorry - I should have asked. Have you always been slim, or did you lose weight recently? If you lost weight recently is your Doctor comfortable there is no unwelcome reason for that?
If all is well, I'd suggest you start simply increasing your portions just a bit, but if that doesn't work, then add a bit more protein, then if that still doesn't work, up the fats.
Whilst many find losing weight tricky, some find gaining it just as hard. Good luck with it all.