Why do I see different IP-address even when I'm using the same PC?
Most probably you accessing the Internet with the dynamic IP, or by one of the following reasons.
Different IP on your favourite website
If your IP is not dynamic, but you face such a problem it could signal that your account at that website probably was used by another person. In other words, was probably stolen by another person. If so, you have to change the password of your account as fast as possible. It is also recommended changing the password from your e-mail account which is related to the website account. To protect yourself fully, it's also good to change all the passwords you use to access your PC and other critical for you accounts. Keep all those passwords private.
You can also contact the administration of the website and post a report that your account was stolen. Administration could help you understand what's happened and could help you secure your account.
You have been accessed by using different devices
Try to remind for sure, if there was an attempt from your side to visit the website with the different devices. For example, you, probably were accessed the website with your mobile phone or pad, or maybe, you were used some software on another PC, which makes an access to your account at a specific website.
Your PC has several interfaces to access Internet
Modern PCs and laptops have several network interfaces allowing you to access the Internet. For example, my laptop has Ethernet and Wi-Fi adapters which I could use at the same time. Through each adapter, it's possible to access an Internet with the different providers or obtain different IP addresses from the same provider.
You have several wireless access points
For today around us a lot of wireless networks, which we can use to access the Internet. Alot lot of them is free to use. Your PC maybe be configured in a way to connect automatically to one of available wireless network if you have a problem with current connection, and another network is available. It means, if one point disappears, your computer automatically could pick-up another one and your connection becomes established through a different ISP and with a different IP-address.