My dog is acting strange like she is sick but I don't know if she has a tumor or stomach virus or other diseaes dogs can get.I know something is wrong but how can I tell what it is

Absolutely a no brainer, nat, you take your dog to the vet so she can be diagnosed properly. How is she acting? Is she eating, drinking? What tells you it is something wrong with your stomach?

No one can tell you this but a qualified vet and if you feel your dog is really ill, the vet must be visited.

Tick

I'm sure she is not acting herself, but did you see her eat something that could make her sick? Was this sickness brought on quickly? Or did it slowly appear?

Parasites and viruses can be tested through a fecal. Some viruses need a blood sample. Feel around in her stomach. If you feel some hard, then it's a tumor.

You need to go to the vet if you feel your dog is not acting right. There is no way of anyone knowing what is wrong with y

I'm sure she is not acting herself, but did you see her eat something that could make her sick? Was this sickness brought on quickly? Or did it slowly appear?

Parasites and viruses can be tested through a fecal. Some viruses need a blood sample. Feel around in her stomach. If you feel some hard, then it's a tumor.

You need to go to the vet if you feel your dog is not acting right. There is no way of anyone knowing what is wrong with your dog.

I'm just going to be the echo here, You know your dog better than anyone else, if you feel that something is just not right then it probably isn't.
Best Wishes

I'll make it unanimous. It's time to go to the vet. Obviously you know something is wrong, but no one on the internet can tell you what it is. Only a vet can, and that should be where you're headed to.

Good luck.