When i push on my tailbone,both legs almost collapse under me and are real weak for a few seconds,what does that mean?

Injury?: Do you know of a cause (fall, auto accident, etc) that might have broken your coccyx? W/out clear cause it's less likely broken, but this indicates some damage's been sustained. See a doctor (at an ER if it's likely to be broken) for eval ASAP - don't press or poke at it to see how it is. Curiosity can paralyze the cat in this case. If nothing big happened, still have it evaluated if it lasts.