What happened and what do I do???

My son is 5 weeks old, for about a week he's had a stuffy nose and nothing else until just 20 min ago I was breast feeding him and he got really sweaty and hot but it didn't last even a minute.. I tried to take his temperature as fast I could before it went away, the soother went to 30 and the thermometer in his arm pit went to 35 but by that time his skin already went way down and there was no more sweat.. Anybody know what this could be? New mom freaking out..