I agree, you're not being a baby or a whiner at all. No matter how tough you are pain will eventually wear you down and can be incredibly daunting. If you are working full time it can seem like even getting out of bed and hauling yourself to work is insurmountable, let alone having to be there all day, then drag yourself home and know you get to do it the next day.
For me this has been going on for several years, it was slow starting out but once it really got rolling I've had several spells where I thought I was going to have to change or quit my job. I drive for a living, but in addition to all the sitting I get to do physical stuff like strapping down people in wheelchairs, pushing those same people (some are incredibly heavy electric wheelchairs). With the sitting I get stiff so then I'm doing the zombie when I have to get up and move around. The pain can swing in intensity, and also location, which can lead to a lot of frustration and in some ways it can be very humiliating. Yes, I know I'm limping, no, I didn't twist my ankle; yes, I was limping on the other foot yesterday, no I'm not bullshitting you that it moved to somewhere else.
One of my coworkers has RA and a fulltime desk job, she knows I have PsA so we are basically our own support group at work, though a couple others have PsA and we all kind of offer a shoulder to (gently! always gently!) lean on. I'm outspoken as to what I'm going through, and willing to listen and offer ideas or insights to coworkers, so that they know what is going on, and with my coworkers who are dealing with similar issues so they know they aren't alone and there is someone who understands how it can be.
As for work itself, I would suggest that first you talk to your human resources department if you have one. Explain what is going on, be upfront and honest. You should be able to qualify for the Family Medical Leave Act for time off if need be, and also that can help protect you if you need time off. Disability is a broad term, you are disabled from the aspect of you have a disease that can definitely impact your work and ability to perform, so it might not be unreasonable to see if HR can see about ways to make things a bit easier for you. If you have a good rapport with your supervisors and feel comfortable doing so, then I'd talk to them, too. I'm sure they are well aware something is going on, so it's not like it will be a bolt out of the blue.
Maybe once you are on Remicade it will help, and you will start feeling better. I'm on it and it's made working so much easier. I feel a lot better and am able to cope with the pain and fatigue so much better. It doesn't work for everyone, and can take a while to work, but before making any huge decisions on your job I'd see how it works for you. It could end up that you feel much more like your old self!
Good luck! Don't give up!