What did I do wrong...?

I recently purchased an eCigar and got the idea to make my own eLiquid to fill the cigar with. After a little bit of research I found out that all of the eLiquids are Glycerin based and that it is very easy to mix glycerin and herb. Below you will find the steps I took to try and mix glycerin and herb and if you would please someone let me know where I messed up.

Equipment Used:

1x 50ml Pyrex Glass Beaker
1x Rubber Stopper for Beaker
1x USB Powered Coffee Warmer Hot plate (max temperature around 140 F)
1x 100% VSP Vegetable Glycerin
1x Dime bag of herb

Steps:

1. Washed and cleaned all equipment with an iodine sanitizer
2. Fill beaker with about 10ml of glycerin and a dime bag of ground herb.
3. Sealed beaker with rubber stopper and placed on coffee warmer hot plate. Only removing the mixture maybe 3 times a time to shake the mixture slightly.

I let the mixture sit on the hot plate for 7 days. After about the 2nd day the glycerin started turning green (as expected) and had a very strong herb based smell to it. On the 6th day I took a small sample (about 5 drops worth) of the mixture and it had a very sweet taste and gave me a slight buzz. On the 8th day I got slightly impatient and decided to move the beaker to my stove. I set the heat on the stove at the mark between low and medium low and went to watch TV. After 60 minutes I came back to check the mixture and the color had gone from a greenish color to a dark brown. After sampling the mixture it had an awful taste and no longer gave any type of buzz.

My knowledge on this subject is limited, but from what I do understand the heat helps release the THC from the plant. Why would adding slightly more heat cause the mixture to no longer have any effect. I do not think the heat was too high as I have been able to make butter before at much higher temperatures. Any information that anyone can provide would be greatly appreciated.

Thanks,

whit3fir3