Tyres are reinforced internally, for toughness.....and this
reinforcement maintains the tyre shape when inflated (or else it'll
bloat up round round like a balloon).
So if there is a lump, it probably means that the reinforcement at
that point has weakend/broken down. This is not the first time I've
seen lumps in tyres (I've experienced it before and so have my
friends).
Yes, its very dangerous to drive fast with such a tyre condition.
Luck is on your side, however.....because its the rear tyre thats
affected. This means that if you're not going at high speeds (I
recommend 50 km/hr max.....yeah, yeah, I know its crazy and
excruciating, but its only a few minutes of your life), a burst
will do nothing much....just maybe slightly out of control.
To be safer, a better way is just change the broken tyre to your
spare....then carry the broken one in your booth for inspection by
the manufacturer.
However, you must know that there are no warranties of this sort
for tyres (they come under wear-and-tear parts).....so even if the
manufacturer inspects it, they'll probably ask you to buy a new
"set".
To balance off, we always have change two tyres at the same time (a
"set" ), never just one (unless all the other tyres are still
really new). Usually, they'll put the new ones in front. The only
thing is that you'll have to buy some tyre polish to make them all
look the same colour (if you're fussy about it).
Safety first ah.....money second.
Great sex third. 
