First, stress is not a mental illness and can't be treated by medication. Anxiety is a mental disorder. It's important to be able to tell the difference, because stress requires you to learn not get stressed out by something specific while anxiety is free-floating and spreads if not controlled. You can't avoid stress in life, and everyone feels it at some point. Not everyone suffers from anxiety, which is chronic and usually makes no real sense. As for CBT, theoretically, if that works, you're cured and don't need medication anymore. Therapy and medication both have pretty dismal rates of success, but that ignores that if one doesn't work you can try another. Studies don't and probably can't account for that. As for your Zoloft, if your stress at work is a temporary situation because things got harder, that will pass when you get past this part. Medications aren't magic pills. While they can mitigate symptoms of a mental disorder, they aren't happy pills that get rid of all the difficult things that are going to happen in life. Which brings us back to the question -- are you suffering from stress or has the drug stopped working for your anxiety requiring a higher dose? You can't look at the drug as a happy pill because if you do, every time you're confronted with a difficult stretch you'll ask for higher doses until you max out and the drug isn't your friend anymore. As for the proper dose for you, Zoloft users usually take more than what you were taking, so yeah, you can go higher. The proper person to ask about that is your psychiatrist, who can evaluate whether your mental illness is getting worse, the drug isn't working anymore, or you're just facing a temporary harder time like anyone else does. A general doc won't necessarily be able to do this evaluation, as they might know a little bit from the drug manufacturer about the drug but haven't studied any psychology and very little psychopharmacology and so have less experience and education to allow them to make an accurate evaluation. This isn't to say psychiatrists are all that great, most aren't, but they are the specialists and this is all they do. But again, everything I'm saying or anyone can tell you is based on our own experiences and generalizations, not on you as an individual, and we do vary a lot in how our illnesses manifest and how they respond to treatment. But if you still need Zoloft, the CBT you did didn't take, if it had you wouldn't have needed the med anymore. I know this is very hard to grasp for us, but when therapy truly works we don't need meds anymore. If we still do, it might be helping, but it hasn't cured us and we still need to keep looking for that even if we never find it. This is a long way of saying, the recommendation is, do what's not only going to make you feel better for now but for always. If it's just some added stress, you might want to see if you can overcome it and wait it out. If it's the anxiety, the drug isn't working anymore at the current dosage and you do need to increase it but sometimes drugs stop working. All the best.
How are you feeling now? I'm really happy to hear that you found a medication that works well for you. It can make all the difference. 50mg is a very low dose of Zoloft. The average dose of those who take it is much higher. But it worked great for you until recently. I would stick with the increase you are at for now and give it a couple more weeks and then increase if you are still feeling the panicky bad feelings. You know yourself and you know what feels 'right'. While stress is normal, you aren't describing that because you are a patient that is treated for anxiety. You know the difference. But do give this a bit of time to work too.
Is this a situational thing at work? Or is this just the day in and day out job?