Guy (27) was super into me (f 19) for a week straight then flipped

Quick backstory- we met a year ago, hooked up and hit it off. texted for a bit but I didn't go on our first date because I got back together with my ex. Fast forward to last week, we texted and met up that night. Amazing chemistry, then we went out on a date the next morning! Date was perfect. Then he texted me nonstop for 4 days straight about how much he liked me, how excited he was for our next date (friday) then friday rolls around and he cancels 2 hours before. then meets up with me that night while I am out with my friends and acts STRANGE. like he was on drugs.. I asked him what happened and he was very vague. said he wanted to make it up to me next week. now he hasn't texted me since yesterday and just posted a snap story with another girl (could be his sister) Why did he change overnight? I am so confused. Sorry it is so long