https://tmep.uspto.gov/RDMS/TMEP/current#/current/TMEP-900d1...
Just like patents have a concept of "prior art," trademarks have a concept of "first use in commerce."
If you can demonstrate that you were the first to use the trademark, you have less to worry about (but nothing's guaranteed until litigated).
HOWEVER, if the party that registered the trademark can demonstrate that they used it prior to you, regardless of when they registered it, then they may have the upper hand.
This is probably not DIY territory; consult a lawyer knowledgable about trademark law to help you determine how to respond.
They may, in fact, recommend that you countersue for infringement. Might not be worth your time to pursue, but at least in theory you could claim that they're infringing on your trademark, because the U.S. follows a "first to use" rather than a "first to file" trademark system.