Ceiling light fixture

I have a juction box in the ceiling. There are four 2/12 wires comming in. #1 comes from the electrical panel #2 goes to a light switch #3 goes to a wall outlet #4 goes to track lighting 5 feet away. Everything was working as expected. The switch, lights and the outlet showed proper wiring when tested. When making the connections in the juction box, I matched up the colors of wire #1 and #3 with a jumper from each. I used a wire nut to make the connections. When I turned the circut on at the panel everything seemed fine. Then the black wire started smoking at the connection I made in the junction box. I immediately turned the breaker off! When I looked at the wires the nut was melted. The breaker seemed OK although it did not trip. What might have caused this? Everything had been working for days. Should I try it again? What, if anything, do I need to replace? Thanks

Hello and thanks for ypur help. I am replacing a dining room ceiling fixture in 45 year old house, operated by one on/off sitch. Old fixture had only two wires connected - one to red and one to white. However, in the box it is not one white wire but a group of three white wires (and only one red). Also, there is a group of three black wires (previously not connected to light fixture). Also I find no ground wires. Instructions of new fixture say connect ribbed wire to white and smooth to black. And conect both ground wires to green screw in mounting bracket. SOOO, being untrained, my "Plan A" is to connect ribbed to group of three white, and smooth to red. I expect outlet box is grounded so I will attach fixture ground to outlet box. Is PLAN A ok? is it safe? How best to connect fixture ground to outlet box. I do not own a circuit tester or phase tester, but I coould figure out how to use one if if neededd. Thank you for your good help.