Minecraft is asking me to buy minecraft even though ive already done that

It all started yesterday when I was trying to play on the hypixel server and Minecraft kept on telling me that I had an ivalid session ID. After trying to fix it for a while, I gave up. Today, the same thing happened, so I uninstalled minecraft and re-installed it. Then, I logged out and restarted my computer. However, when I logged back in, it was telling me that I could only play the demo version of minecraft because I hadn't bought it, yet when I go to the website it says I have bought it. I also tried on another computer and minecraft was working fine when i used the account.