battery changing issues

OK I changed the 12 volt battery myself - man that was some kind of herculean effort to get that done! How much stuff can Toyota put in the way of the battery that you have to disassemble just to get to the battery???

Finally after a back-breaking effort I got the battery changed and hooked everything up. Success! the car started right up. But then after about 2 miles of driving, the red triangle warning light came on and also the check engine light. The car seemed to be running smooth so I kept going. Then the red light went out but the check engine light stayed on. The next day the check engine light was gone. Then everything was fine until a week ago when I started up the car and the check engine light went back on and the digital display panel went out!! Blank. Couldn't see my speedometer, my odometer, my gas gage, nothing. but still I pushed the button and the hit the gas and I could hear that the car was on so I drove without the display panel on. Then I got home and I pushed the power button and the car wouldn't turn off! It was Sunday night so no mechanics were open so I waited a half hour and then pushed the button and the car turned off. I checked the car a couple times and it was the same thing... check engine light on, and no display panel. Then Monday morning i went out to the car and started it up and voila... the display panel is back on and no check engine light. I guess the car is OK but what the hell happened??? All i did was change the battery!!