How long have you been taking it and what is the doseage? I seriously doubt it would raise your BS as it is suppose to help lower it. You said you didn't eat a lot but did you eat things that would raise your BS that high?
If this is all new to you do a search on low carb and low glycemic foods. It's not only the portions we eat but what we eat. Metformin will only work if you are eating the proper diet and get some exercise. If the Metformin doesn't lower your BS your Dr. may have to add something else with it.
When were you diagnosed and what was your A1c level before being put on the Metformin? It does take awhile for it to lower your numbers.