Can the district attorney/ state press new charges based on prior charges that were dismissed?

I recently discovered I have a warrant issued for me in another state. I called the warrant dept in that state and was told I'm being charged with harassment. Apparently in 09 my ex filed a complaint with the local authorities for harassment which is completely false. The only reason she did it was due to the fact she found out I was with someone else and wasn't to happy about it. She keeps apologizing and saying she didn't mean for it to go that far. I also found out the only probable cause the P.D and D.A. have for this case is from a domestic case between her and I in 2007. That case was dismissed by the judge and wasn't even in the same state I have the warrant in now. My question is can the D.A. press charges and issue a warrant based on a different case...is that considered probable cause? Or am I screwed and need a lawyer since the state is trying to prosecute now?