My mum has a coldsore from stress, i shared sweets with her + took a bite from the other end of 1 she had bitten but not where she bit, will i get 1?

Coldsores: Coldsores are a reactivation of a previous viral infection that hides in nerve endings. Some people can get recurrent ones especially when stressed. It would be very unusual to catch it this way. If area is dried up , makes it really unlikely. Most people have a primary infection with the virus as a young child, but everybody responds different under stress

Contagious cold sore: You should be very careful NOT to exchange any saliva which could be infectious..So no shared food, straws, cigarettes. Also avoid behaviors like kissing. From what you describe, I doubt you got a 'mouthful' of germs but do be careful. There's a chance you've already got it though if your mum has it -ever have an outbreak? Virus can also be shed WITHOUT an obvious cold sore-so stick to rules above