Conspicuous posting on leased premises?

All occupants of a space are responsible for the rent. Ask your roommate if s/he paid rent. If not, then you need to pay rent in three days or you will have to move out. Otherwise, you will be evicted and that makes it hard to rent elsewhere.

So first step is to talk to your roommate. They could have ripped you off, bounced a check, etc. If your roommate didn't pay and you don't make good on it, you all have to leave. If your roommate can't prove to you that they paid the rent, then it needs to get paid. Otherwise, you have to move. If you prepaid some rent, you will be entitled to a refund less whatever the rent is for the time you were there that hasn't been paid.

That's the law. What your landlord does elsewhere (which is rumor and gossip) is legally irrelevant. What is relevant is whether or not your rent has been paid.

Likely you and your roommate are on the lease together. This means that either party is responsible for the full rent. The landlord does not care how you split things up, he just want full rent each month. If either of you fail to pay, you can both be evicted.

The specific rules vary by state. In some places, the roommate can be evicted SEPARATELY, and you can be allowed to stay if you can find a suitable replacement for the deadbeat. In MOST places, however, the roommates are jointly responsible for the entire rent and both can be evicted for non-payment.