Should I be worried? 6 months post gastric sleeve and I've gained weight and have pain.

At six months after a sleeve gastrectomy, gaining back some weight and experiencing bloating, gas, or abdominal discomfort is not typical and should not be ignored. While a 15-pound gain is not uncommon over a longer timeframe, such a change this early on—especially when combined with tenderness and pain—deserves careful evaluation.
There are several possible explanations. In some patients, symptoms like bloating and discomfort can be related to reflux, dietary intolerances, or changes in eating habits. However, abdominal pain and tenderness may also suggest an anatomical issue with the sleeve such as narrowing, ulceration, or rarely, a complication like a hernia. These issues cannot be diagnosed without proper testing.
I recommend contacting your bariatric surgeon promptly to discuss your symptoms. In most cases, your surgeon will arrange an upper endoscopy or imaging study to rule out anatomical problems. Even if the cause turns out to be functional (such as reflux or dietary intolerance), early evaluation is important to ensure nothing more serious is being missed.
The encouraging news is that most causes of these symptoms can be managed effectively once identified. Please reach out to your bariatric team as soon as possible so they can guide you safely and help you get back on track with both your comfort and your weight loss.

Hello, I recommend that you go to your certified doctor or nutritionist so you can find guidance and answers to what diet is best for you., it depends in your metabolisms and how your body reacts to different types of diet plans, talk to your doctor to find the best solution for you. 
Best Regards
Do not get discouraged in the search of options to improve your health.
My Best Regards