horribly slow internet speeds

would someone be able to help me rectify what is wrong with my home internet. I am paying for the 100Mb connection, however when I run speed test the highest I have seen in the last week is 13Mb. I have reset the modem numerous times, I have reconnected every device to ensure the most up to date settings are in there. I also disconnected every device except one and the 13Mb is the best I could see. Considering I constantly stream Hulu, and other video services with multiple phones and tablets connected, with never an issue. Now I can only stream data onto one device and the video will buffer if any of us get on facebook.