I had sexual inter course with my girlfriend yesterday but the condom slipped not all the way off but halfway off any i immediately stopped is there any chance i could have an std or sti?

Only if germ present: Condoms are barriers, like a raincoat. They protect what they cover. Any person using a condom can still catch STD's on the uncovered body parts, such as the pubic area and the base of the penis. Even when wearing a condom, a person can transmit things like syphilis, herpes, pubic lice, etc., from his non-covered areas. That said, condoms are great and should be used every time.