Welcome to the forum
Certainly the gastro effects are not unusual when starting on metformin which should be introduced gradually with 1 tablet at first for at least a week then increasing if needed. Make sure you are staying hydrated.
High blood glucose can cause tiredness and thirst so that could be the explanation, it is what often sends people to the GP and that is when they get diagnosed.
Were you told what your HbA1C is as that will indicate how far you are into the diabetes zone and give an idea as to how much work you will need to do with dietary changes as the metformin is not a magic bullet and can only help the body use the insulin it produces more effectively.
Losing weight if you need to will help reduce your blood glucose, as will increasing exercise if you can.
This link may help you with some ideas for modifying your diet as it is a low carbohydrate approach that many have found successful.
https://lowcarbfreshwell.co.uk/
Other people find a low calorie or shakes - based regime will give them a kick start, whatever you choose needs to be enjoyable otherwise it is not sustainable.
The Learning zone (orange tab at the top ) also have useful information and links to recipes and menus.
As I said how much you need to do depends on your HbA1C, if not too high then some modest changes may be all that is needed.
