Why is my computer suddenly running slow?

Lenovo Ideapad y510p
Specs:
i7 with 2.4 ghz
1 TB HDD
Nvidia Gforce gtx 750
About a year and a half old

I left for a trip during the holidays and took my laptop which is usually hooked up to a monitor for a dual monitor display. Programs normally run great and games like Skyrim (somewhat intensive) can run on high settings. However, I unplugged my HDMI from my other monitor and upon bootup when I had reached my destination, I discovered that the laptop was running differently. Everything was greatly zoomed in (which doesn't bother me) and programs ran slow (which greatly bothers me). I tried playing normal DVDs in my ROM and they would randomly freeze up and the media player would become unresponsive. The default screensaver (video that activates upon an idle time of one minute) that Lenovo gave me now gets very pixelated and unclear. I tried to run Skyrim just as a test and it is beyond laggy.
The problem may fix itself when I replug my computer into the other monitor in a few days and the whole system runs as it normally does.
I also upgraded to Windows 10 recently and that may be the problem. If this persists I will just restore my laptop to factory settings back to 8.1 on which everything always worked.
Any comments, ideas, or suggestions? I really don't want to have to rest the entire laptop as doing so would be a huge pain.

Unplugged the HDMI cable, then the problem.
I would say it is the Video Driver needs to be updated (or even downgraded) to the new monitor's needs.
Especially since you made the Mistake of getting the Windows 10.
That is most likely the real problem here.
Your machine was optimized for the prior OS. Going back to W8.1 would be the thing to do. OR spend some time finding new Drivers for the W10.0 OS, if there are any.

My suggestion is Roll-Back to the old Operating System.
Wait until you buy your new machine with a Windows 10 OS.