Good question Samantha! I also have curly hair that is sometimes wavy. The structure of hair (straight, wavy, curly) is determined by both the hair follicle (the pores on your scalp where hair grows from) and the molecular structure of the hair itself. People with curly hair, like you and I, have oval shaped follicles, while people with straight hair have round follicles.

The flatter the follicle, the more curly the hair. Hair is made of dead cells that contain a lot of a protein called keratin. Keratin has many cystine amino acids that form a strong bond when they come into contact making a helical structure. Curly hair has more disulfide bonds because the flatter follicle forces the cysteines closer together and thus makes them more likely to form bonds. You can use chemicals to break these bonds and straighten your hair and other chemicals to create more of the bonds. Other external factors can influence the number of bonds, like the shampoo you use, or humidity. I believe it is most likely that one of these external factors changed to make your hair less curly, although hair follicle shape can sometimes change with age.

If you want more curly hair I would suggest not shampooing your hair as often.