Hi irish maeve
Cutting out all potatoes/bread/pasta etc is very good. To put on weight simply eat more fat and protein. Meat, fish, butter, double cream, full fat milk, hard cheese, mayonnaise. Aubergines
If you have always been slim I wonder if you might be LADA (Early T1). The only way to establish this is with a GAD test through hour GP, but they are expensive and gps don't like doing it. Have a read through some of the forum posts in the LADA section. If you then think you have some of the symptoms see your GP and tel him/ her why you think you might be LADA.