Sounds like a strain. The fact that it got progressively worse throughout the day could have been from inflammation. Definitely don't try to work through the pain if it's a shoulder injury. For now, try resting it, maybe use some ice and compression to help with the pain. Elevation of the injured area helps too. When you don't feel any pain anymore, start light and go from there.
Here's something you might want to read. It may not apply to you in this specific situation, but it's a good read nonetheless:
If the pain starts to subside over time, I would just wait for it to heal up before hitting the weights again. If for some reason it starts to get worse even though you're not agitating it, I would seek a medical opinion.
The RICE Method
Injuries to the AC joint (which may or may not be the case)
Front Deltoid Strains
- A possibility, since the front deltoid comes into play quite often during exercise.