Dispute over Special Assessment

I own a condo unit in NE. Earlier this summer, the association voted to build a rooftop deck. Each unit was assessed a special assessment based on the size of their unit. Mine was about $533. Not a large amount, but it came at a bad time. We were allowed to make payments for up to a year. No amount was ever established that had to be paid per month or anything. I paid $50 one month, nothing the next, and this month I got my statement with a late fee of $25 because I hadn't made a payment on the special assessment.

I complained to the board, who basically said "too bad". So this month I paid another $50 and I'll continue to do so. I'm worried that they'll try to place a lien on my unit now that I've complained so much. I"m not even sure what a lien is - but I don't like the sound of it!