What could the ball of cloud I saw be?
It's hard to visualize exactly what you saw, but typically in convective clouds, there is such significant motion that oftentimes very chaotic and varying cloud shapes will develop as a result, and they may move quite quickly due to the strong winds.  It appears the term pannus may be used for these, though I'm more familiar with calling them scud.  You mentioned rain, it's northern hemisphere summer, so these cumulus-convection byproducts sounds quite likely.

Around other storm chasers/meteorologists, I've also heard these types of clouds called debris clouds... which is a rather apropos description given that such clouds are often leftovers from earlier storm evolution (perhaps even remnants of old wall clouds if in a situation with supercells)... but using that term might bring confusion to others because tornado debris can also be called a debris cloud.  This page appears to exemplify the confusion, as the picture appears to include remnant clouds from earlier storms in the foreground while the description gives the tornadic definition.  So we probably should avoid this term.

There's no bad gas in thunderstorms... what you see in meteorological clouds is liquid or ice water (all clouds form around tiny bits of dust and such, but this wouldn't affect appearance).  Occasionally smoke or such may be ingested in a cloud, but even in situations where large fires and volcanic eruptions produce clouds, the forced moist updraft region will often look quite similar to a typical cumulus cloud.