Hi and welcome,
I too am pretty small and light around 5'4" and 100lbs and one thing is for certain, normal S size suits won't fit you.
I tried on quite a few different 7mm suits for cold water and the one that will definitely fit is the Bare Alpine Junior, just compare yourself to the size chart at the Bare website and see which one will fit.
I ended up getting that one and like it, I've been diving it in Northern California and could last about an hour in the water, not toasty warm but manageable (48-52F).
Diving colder than that and looking at it now I'm going to buy a drysuit, that's for sure.
Another option would be a women's suit, I tried those on as well and they do fit if you can live with little breast extensions on some of them!
Also there is the Pinnacle Elite suit which is for cold water as well and available in XS, which may fit, I never tried that one on personally though.
Hope that helps.