You've received some great advice so far, certainly try to follow it.
To answer your question: Yes, I've done this and no, you wont be back in full withdrawal at all. The worst you'll feel is a twinge of pain in the lower back and a little down for 3/4 days but most of that psychological misery, if indeed any, will be due to you feeling as though you've failed yourself. Please don't worry yourself. However you must get to the root of what caused you to use again. Something triggered you to use and its imperative you attend to whatever that circumstance.
The worst thing you can do is to hate yourself for this buddy. You had a little slip - as did I - and that's all it was: a slip. That is not a full blown relapse.
My own experience is not dissimilar, though I'll need to give a little background info. I was almost three months off heroin following a very intense at home cold turkey which I began around last February 5th. On the 15th of February I had a seizure due to extreme dehydration and only getting a few hours of sleep over that 10 day period. My brother found me foaming at the mouth in the porch of my parents house and after calling the ambulance I was admitted to hospital where I had a psychotic episode involving visual and auditory hallucinations.
They kept me in for 12 days in total, gave me an anti-psychotic in the evening which got me to sleep and killed the hallucinations and 30mgs of valium a day for the horrendous anxiety - but heres the part I'm a bit pissed at. By day 8 I was feeling fantastic. Back to my good old self again...however, as my mind began to clear I realised they had been giving me 20mls of methadone since day 5. I began to get very angry as I was on methadone for 2 years from 2013-2015 and everyday of that time was spent in hellish depression. Predictably enough, by day 11, as the methadone was really sinking into my system, I was absolutely miserable.
Anyway, my father came to visit me one evening and we came to an agreement where I promised him that I'd stay on the methadone on two conditions: that it will absolutely not be raised above 20mls, and that I will initially stay on 20mls for two weeks but then I will be decreasing 5mls a week there after. I successfully did this and jumped off at 5mls last Sunday.
Since the doctor provided me with some medications (20mgs Valium daily - barely takes the edge off but certainly beats nothing, 7.5mgs Zopiclone/Zimmovane nightly for sleep - useless, and 10mgs Cysterin for the sweats - which work wonderfully) in order to take the edge of the potential withdrawal, the physical withdrawal has been greatly reduced...so far at least - remember, I'm only off the methadone three days) but the depression has been BRUTAL. I ended up having my own little slip yesterday with a .2 of North Inner City Dublins finest brown. It took the depression away for the day and allowed me to get some things done but its not something I want to do again. I whole heartedly regret the entire experience.
Anyway, I have quite a bit of experience spanning 7 years but I feel like I've ranted away here but please do ask me anything you want and I'll be here to answer anything you need to know as helping people out here on BL keeps my mind occupied and (strangely enough) away from scoring.
How are you feeling today Snitts? Hope to hear from you. Again, please don't hate yourself - you're not a bad person, you're merely still psychologically habituated to the ritual. I'll bet you didnt even particularly enjoy it, right?
Anyway, congrats on 20 days and keep pushing forward - you're nearing a major turning point. PM me anytime you need some support.
Peace, Love and Light friend.
PS: I'll update tomorrow and the day after to report on whether or not I do feel any worse.