Why do I have increased pain and pressure in the tooth after undergoing root canal treatment?

Hi doctor,

I did a root canal on the upper first molar tooth before three weeks. I had sensitivity towards cold and sweets for years. The tooth was slightly loose. So I did a root canal on it. The second sitting was done before a week. I started to have pressure on the tooth after two days. I consulted the dentist, and he told me that the seal has got broken. The doctor said to me that the tooth should be pain-free for a weekend to have a fixing over it. But, I had more pain than before doing a root canal. The pain was terrible. Now, I have pain, pressure and it hurts even for a light pressure on the tooth. I do not have sensitivity. Now, it has been two days, and still, I have pain over the tooth. Please help.