Tooth pain after crown

I had to visit a new dentist when my old dentist moved her practice. A portion of my far rear left tooth chipped off while eating food. I went to the dentist who instructed me that a crown on that bottom tooth was neccessary. A temporary and then permanent crown was put in and I ended up going back to have three other small cavities filled after that appointment. Since then, I have had pain associated with the crowned tooth. It is sensitive to cold and for the past three weeks I have been living on Motrin to stop the pain. The pain comes at random times during the day and I often wake up in the middle of the night. At night time it feels like all my teeth are hurting. I often have pain on my lower left teeth and upper left teeth as well during the day. I went back to my dentist who said my problem is that I am grinding or clenching my teeth at night and this is responsible for my pain. I am not sure what to do at this point as I have never had any problem prior to this work being done. I am also concerned with this explanation as I have shooting pain on this crowned tooth throughout the day. Sometime the pain can be really bad. I have not seen any improvement over the past weeks. He said to purchase a mouthguard and give it two weeks and the pain should subside. Could it be that grinding my teeth could cause just that one tooth to hurt so bad? I am not so optimistic and would like some advice. Thanks.