Unexpected drugging, Could use some help or advice with this paranoia

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So I've been on probation for about a year now and am very close to getting off and leaving it all behind. I am on probation for possession of marijuana and haven't used it at all the whole time I've been on probation and have been trying as hard as possible to really make my life better and clean myself up. Today while I was at work one of the waiters asked me to fill up a blender with some ice cream and he would bring me back a special milkshake. I didn't really think twice about this because I really would never expect someone to make a shake with an illegal substance in it while they were at work or in a public restaurant like this. Anyways I continued working because we were pretty busy and eventually the shake was on my station. I took one sip of it and instantly new that there was some marijuana in it! I instantly waved him over and asked him about it and then told him that I couldn't have it. Anyways I was wondering if anyone else here has had an experience like this? Or if anyone here knew whether or not I would fail a drug test in a week or so from taking a small sip of a milkshake with some marijuana in it? I'm pretty nervous and paranoid about this situation because I've been working so hard and am so close to getting off probation. I really didn't need something like this to rack my nerves more than they have been the past 11 months. The worst part is that it wasn't something that was a conscious decision by me or something that I meant to do. I'm just hoping that I will be okay with this situation because I really don't need to go to jail or anything because of this small fluke.