Do I hurt my ears by listening to music at...

If you have tinnitus, 75dB is not the cause. Unless it is 75dB real continuous and for hours. But even then that will usually only be a short term loss. Like when you get out of the car after a long trip, the road noise means you need a few hours to get your fine hearing back.

At 18 you should only be hearing ringing after a 105dB rock concert. Even then it should be gone by the next day. But tinnitus can have many different causes, and even be totally unrelated to loud sound exposure. Drugs like aspirin, and even ibuprofen can cause it, although again it should take a lot over a long time. I gave myself tinnitus one time by taking a boatload of aspirin for a bad burn. Point being, do a little research, maybe even see a doctor. 

The OSHA standard is 85 dB weighted average over an 8 hour work day. Employers are required to have hearing conservation programs if workers are continuously exposed to levels above that. 70 dB for the time period you are talking about should not be an issue. 

In my view it’s quite unlikely you have caused damage, let alone tinnitus, with your relatively modest listening habits.

There are a lot of covarying factors involved with tinnitus; some psychological, and of course, there can be purely physical underlying causes that you would want ruled out (such as acoustic neuroma).

But don’t listen to me, seek out a medical professional if you’re concerned.  

Tinnitus can be caused by many, many things. Unlikely it is listening @ 70db.