I had a similar experience - I was living abroad and I got this killer UTI that just wouldn't go away...I went through round after round after round of antibiotics, and finally got rid of it, and about 6 mos later this crushing fatigue hit me, and that's when my weight gain really began.
What helped me the most was to go on a really strict anti-candida program (antibiotics build yeast, and yeast could be implicated in your IBS) - I didn't touch any sugar or refined carbs for 60 days and it TOTALLY helped, and I even lost weight (though I put it back on later, as my PCOS symptoms got worse!

).
You might also try something to boost adrenals (like siberian ginseng) and something for your liver (like a detox tea with dandelion).
GOOD LUCK! I feel for you - I struggled for a long time with fatigue; thank god it's better.
Lucie