This doesn't deal with Tamiflu, per se, but my wife is a hospital pharmacist. She called me yesterday from work to say she wasn't feeling well (I had been sick for three days with some virulent GI problem so I suggested she head home). Well, she didn't until she thought she couldn't make it so I suggested she see a doctor there at the hospital. Long story short, she has both A and B strains of influenza and had taken the vaccine. They repeated the swab after the first positive because she had taken the vaccine. So whether I had influenza or not, I don't know for certain, but she did. Which, of course, is in consonance with the above post on vaccines not working. Viruses mutate, or those who decided which flu viruses to choose as the likely pathogens in a given year's vaccine make imprecise choices.
Actual influenza won't make you have GI problems. You just probably have another bug going around, which it is. There's only one kind of "flu" that makes you have stomach problems and that's the so-called "Fiji Flu" which is common only to Oceania and other Pacific Rim areas and that's basically just locale slang for "I have a stomach virus."
My Mother right now is down with a bad cold with a small fever. So I am sure the next time I come in contact with her she'll give it to me, which is fine. I'm about 4 months overdue for my standard "You will have a cold over Christmas!" cold. It's like clockwork. Either I get it during Christmas or I get it in the Spring.