gas heater

Hello, I'm having trouble with my heater. I'm a novice when it comes to this stuff! It's gas system - as a rule, I don't mess with gas or plumbing so I don't want to tinker too much.
The whole story...first we noticed the house wasn't getting as warm as usual, but the heater was kicking on more than normal. We could hear water running through the pipes, when typically quiet. Next, our gas bill was $100 higher than normal. I tried to bleed all the pipes, figuring air was the problem.
I also noticed the pressure in the system was down to 8 psi. So I let in new water and it shot up to around 40 psi. Too much! So I again bled the system down to around 15 psi. The system now fluxuates between 17 and 12 psi. Not sure if that's normal??
Now a valve (assuming pressure valve?) is leaking slightly. Not sure I fixed the orginal problem either as the heater still doesn't seem as efficient as usual.
Are all these things connected? Did I mess something up? I just don't have a good feeling about! Hoping this something simple that I fix. Want to avoid a costly plumber's bill. Any advice??
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