Personally, I don’t think the issue is if it is a grave matter or not but the question is on intention. This post does not directly apply to your situation since you really hadn’t decided until the last minute and you did make an conscious decision to fast once you had decided to attend Mass. That being said, if you had taken a drink at 9:55 for a 10 AM weekday Mass (Sunday Mass you may still be okay), you should not receive though you should still attend Mass.
I understand you are asking in terms of confession, which it is probably not a mortal sin but should be confessed (as at least one other has mentioned), but in the future it should be something you have at the forefront of your mind as you prepare for Mass. The concept is that you are about to receive the Savior of the universe in the most intimate way imaginable. You should not only prepare you mind and soul for this encounter, but your body as well. The Church has prescribed that fasting for one hour is the minimum sufficient for this preparation. Because of the limitations of our bodies, this one hour minimum should be followed by everyone because you can’t short cut your digestive process. Your mental and spiritual preparation are up to you since each person will vary widely on how they prepare themselves but bodily preparation will have a natural minimum. In my opinion, you should fast shortly after receiving as well but there is no rule for that.
So when it comes to following this rule, it is about intention. Let us assume it is not a grave matter and so can never be a mortal sin, would you then stop worrying about it and just drink tea until you walked in the door? Does it need to be a mortal sin for you to not do it? While your situation is somewhat different since your intention was not to violate the rule, there are many people who just don’t get it.
A similar parallel is fasting. The Church requires fasting during Lent but that does not mean that is the only time you should fast, it just means that is the minimum amount of mortification you should be doing (that and Friday). Many people think fasting on Friday has gone away when really it has just become a personal choice as to what you mortify on Fridays in honor of Christ’s sacrifice, fasting was just the previous minimum. It is all about intention when it comes to these disciplines. The goal should be a desire to fulfill them, not out of a sense of following the law but out of a sense of love for your brother and of God.
I applaud you for trying to get back to a routine of daily Mass, it is not an easy thing to do until it becomes a habit. Just remember why you are going and the fast won’t be an issue. You won’t need a rule because you will want to follow it and maybe even go beyond the minimum out of love for our Lord.
God Bless.