Your pain located in the left upper quadrant of the abdomen could be caused by a variety of conditions. An enlarged spleen (splenomegaly) is certainly something to consider, as it does rest in this position. An enlarged spleen is often the result of liver disease or a blood disorder in which red blood cells are inappropriately destroyed. Infection with EBV causing mononucleosis (mono) is another common cause of splenomegaly.
Another possibility to consider, however, is that the pain you are experiencing is originating from the colon. This is probably the most likely case given that you have also been constipated lately. From the rectum, the colon travels up the left side of the abdomen before turning at the splenic flexure and traveling across the upper part of the abdomen toward the liver. A fair amount of constipation could be putting stress on the wall of the colon in the area of the spleen, as well as contributing to the buildup of intra-intestinal gas in that area. If this is the case, relief of your constipation should help resolve your pain.
Since the cause of your pain is unclear, you should visit your primary care doctor
. With a thorough medical history and physical exam, he or she should be able to come up with an accurate diagnosis and treatment plan.