I've been using my personal hotspot between my iPhone and iPad, but I don't believe I've changed anything with my phone contract, will I be charged on my next bill?

Depends on whether your carrier supports Personal Hotspot & how/if they charge for this feature. Some carriers include Personal Hotspot as part of your data plan, some require it be provisioned on your account. Contact your carrier & see what their requirements are. The fact that you have the Personal Hotspot setting indicates your carrier supports this feature.