Let me start off with I am halfway to my goal weight and within 10 pounds or so of a normal BMI. I am so happy with my progress so far and am so proud of how committed I have been on this journey. I'm finally starting to feel normal, and maybe even a little more confident in my looks. I've always been very insecure with my entire body, and that transferred to me taking a very asexual attitude towards dating. I didn't feel like any man would ever be attracted to me, so I always drifted towards the friend zone.
Anyways, since I've lost some weight I have noticed certain guys that have become a bit "friendlier", and have been border-line or straight out flirting with me.
A certain fellow, we'll call him Mike, is a person I deal with infrequently at work (I am an independant government widget inspector, and he is a manager at a widget building company). Mike and I have a pretty awesome comraderie, he is a good person to deal with and doesn't seem to take my inspection comments personally. Mike has told me I was his favourite widget inspector back when I weighed over 200 lbs.
I saw Mike this past week for the first time since I've lost weight. He was noticablely flirtier this time, asking me about places I like to frequent, and joking (?) around about taking me out for patio season drinks. During the end of my inspection I asked for his office's address/fax/number for my records and he gave me his card. I gave his card back and he said I could keep it and call him sometime, again jokingly (?). Again as I was leaving he called out to call him sometime for drinks (ha ha ha?).
Now, I am totally untrusting of men since I was screwed over pretty consistantly in high school/college by a string of jerkwads who would lie and use me. I guess I was too ugly to commit to. So I don't know whether to take his good humour as he is seriously interested, or if he is just trying to get on my good side to pass inspections and have less hassle at work. I am way to shy to call him, I don't want to offer myself up for rejection again. HELP!

I don't know if this was part of your question or not, but I know I have personally struggled with whether or not a guy likes you because now I've lost weight or not.... but you said he was flirty with you before and you haven't seen him since you've lost the weight so I think it's just you haven't seen each other in quite a while.
Maybe he's worried that if doesn't connect with you now it's going to be a while before he sees you again. 
and it would be a non-issue at work if I dated him. We do not work for the same company, would be kind of be like dating a client. It would be a conflict of interest and he would be get a different inspector. There are plenty of other widgets for me to look at.