Horrible cough

So i've had a bad cough for a few days now and in the last hour i've been coughing so much and hard that my head hurts and my chest is sorta sore. I am currently drinking herbal tea with honey and lemon juice in it but it's not really helping sooth the cough. I am gonna go to the store and buy some over the counter cough medicine but I haven't had a cough in so long so i'm asking what you all think is the best medicine to buy that actually works...at this point idc how expensive it is. I tried niquil last night and it didn't really work. Suggestions or anything please??? I'm dying over here :(