If you have a bladder infection it can take a few days to feel better. It is very serious so you should see a doctor both for the pain and the infection. People can get very ill from bladder infections.
Let me get this right. You are taking Macrobid for a UTI and have been on it for 3 days now? When did the tenderness in you stomach start? This could be a part of the UTI. Have you checked back with your doctor and or their nurse? A UTI is not a small thing. You really should contact your doctor's office and let them know what's going on. It sounds like your appendix but again, contact your doctor's office right away. I've had similar pains before and was told that my appendix may be a part of my problem. Turns out I was just severely constipated and was put on some heavy duty medication that really helped out a lot! Are you having regular BMs? Any nausea?