Gas generally comes from some sort of fermentation or rapid oxidation so something musta become dislodged and while making it's way out it was oxydating rapidly and causing all the gas not to mention that it's prolly toxic matter also thus making you feel sick and nauseous. You mention having done the BP shakes during your juice feast. The BPs are great at what they do but on a juice fast not having other solid matter to help push it out they can become very dehydrating not to mention absorbing a lot of the nutrition from what ever juices you are drinking unless you are very judicious and only do them one hr before or after your juices and then on top of that you hafta drink lots and lots of water because both of the highly absorptive nature of the B&P and to keep things well hydrated to push the stuff out. I mention all this cause if you weren't very judicious as I mentioned before according to Andreas Morrits the B can become plaster like when doing a liquid fast and could very be that you might have formed some kinda small blockage and you just turned loose some of it. Now all of this is simply an educated theory it could just be plain ole detox.
Having said all that the real issue at hand is to get you back on track and the first line of defense is always to up your liquid intake significantly to make sure you are well hydrated and the system is not trying to hold on to all fluids and to keep any toxins diluted and moving out. You can also try cutting back on the cayenne. That stuff is pretty powerful at dislodging mucous/toxins which is a good thing but when more stuff is getting loose than the system can handle it overloads the system with toxins and you can a have a Herxheimer reaction from too many toxins released at once which can make you feel ill nauseous and make you wanna hurl. If you are not really well hydrated it can cause the dry heaves.
You're already hip to doing the water 30 mins pre SWF so keep doing that and just significantly increase your fluid intake both water and lemonade and see how it goes. At least for me and in many cases that have been discussed here the SWF can take 1hr to 1.5 hrs before starting to go through you so empirically that seems to be a norma thing. The other thing I do is if by 1.5 hrs I don't have any out put or significant output from the SWF I'll chug a qt of water.
The MC is also considered a kidney cleanse so it's pretty common to have discomfort there. I suffered from kidney stones pretty bad to the point of being hospitalized a coupla time till I started doing my annual 40 dayers. Now that I mention this one the indications I would have when stuff was beginning to get loose in the kidneys was nausea and my body going into evacuation mode out of every orifice. Is there wasn't anything to vomits or poop out I would just get the dry heaves and stomach cramps so you might have loosened some sediment in your kidneys. Just to reiterate the issue now is to get you back on track and the first line of defense is to increase fluids