You say that you paid the dues for last year and this year, so is the account fully paid by your understanding, and if so did you pay any related late fees? If so, and you think that it is a mistake on their part, show up to court with whatever documentation you have, or call the lawyer listed on the appearance to explain the misunderstanding. If your overtures are not received well, you can go to court and request some contested time where you will be given the opportunity to present evidence. Pay attention to the instructions on the summons and/or notice of claim so you comply with the admittedly lax rules of the small claims court, namely the instruction that if you are going to be presenting any documentation as evidence that you bring three copies of each document.
Best of luck!
Regarding the fact that they gave you til March 1 to pay and cashed the payment on April 28 but still pursued you---do you have the letter stating the March 1 deadline? If you don't have the letter, then it's hard to prove. If you do have the letter, then have you brought the fact that they cashed the check on April 28 to their attorney's attention? Now strictly addressing the remainder of your premise--the HOA has an absolute right to collect through all legal avenues, the monthly dues. Most CC&Rs (the contract between you and the HOA) have attorneys fees clauses, (losing party pays the fees) so I am not surprised that they are pursuing you for the legal fees. If it were me, I'd stem the bleeding and pay (if I could) to avoid more legal fees racking up. If you want to fight, then consult with a local HOA attorney, but again, you'd have to pay them unless they take your case pro bono--but remember, you owe the dues and did not pay them, so I'm not sure of what the defense would be.