Weird thing happened to my 99 Blazer LS this morning

Just like I do every morning I get in my truck and start it. First thing I noticed that didn't work was my door unlocker. I said no problem, battery dead. So I open the door and turn the key and the truck starts up and shuts
off. So I keep turning the key and same thing happens. So I get out look under the hood (not that I know what I'm doing) don't see anything obvious. so I go back in. turn the key to the second position (the one before starting it) and I noticed that it didn't beep to put on my seat beat!. WTF? ok ok I'll turn it off. look under the hood again and don't see anything. go back in my truck and I noticed that my security light is now solid. (always flashes when you get in) WTF right? so I turn the key and my service engine light comes on while in the 2nd position (before starting) never did that. So I go under the hood and disconnect the battery. Waited 15 mins and put it back on. Hopped in my truck and boom it started. Does this sound like something with my computer?
BTW when I started it the first 5 or 6 times it sounded like it started fine , revs went up but when it tried to idle, it shut off. I've been starting it all day today after I took the battery out and seems to be working fine. One more thing, should I be concerned?
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That thing have the factory security system on it?
Doc

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yeah, everything in it is factory

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only changes i've made to the truck was add and amp and a bass tube. Still have the factory deck in it

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