I'm a overweight 17.5 stone 17 y/o and want to lose weight through exercise but everytime i do up to 20 sit upsi get terrible stomach cramps. why so?

Situps: It may just be that your abdominal muscles are weak and not used to doing sit-ups. When you do a very strenuous workout, it is not uncommon to have sore muscles. This usually means you have broken them down and are rebuilding stronger muscles. Start with maybe 10 sit-ups and slowly work your way up to 20 sit-ups over the course of a week or two.