What Causes Stomach Pain After Taking Azithromycin And Flonase?

My 13 yr old son just came from doctor CBC shows bacterial shift - early sinusitis. Begin medrol 4mg dosepak, flonase 59 mcg nasal spray, azithromycin 250mg - 5 day dose pak. Failed to tell dr his legs were hurting and after taking azithromycin flonase, his stomach is hurting. He s been passing trenendous gas prior and drinks dr pepper like water. I can hear n feel his heartbeat in his stomach n his pulse is racing. I can t help but feel something more serious is going on. Should i be alarmed? Temp 99.7, few bites of yogurt helped with stomach pain.