Dear Sarah,

You said:

• Did I commit a sin if I had an impure thought and physical feeling after receiving
  Holy Communion?

No, you didn't. satan [small s] will always strive to screw up our spiritual life, so don't worry about it. Just maintain a warrior-like attitude, spiritually, especially in this culture.

The Rosary is an excellent weapon to use on a regular basis.

Although the bishops encourage the faithful to go to Confession on a regular basis (monthly), the only time one has to go to Confession (before receiving Holy Communion) is when they have committed a mortal sin (like one of a sexual nature). That said, it has to meet the criteria for being a mortal sin.

A mortal sin requires:

• knowledge of what you are doing
• knowledge that it is grievously wrong, and
• giving full consent of the will to the sin.

or for short, for a sin to be mortal you must:

1. really will it
2. know it's wrong, and
3. it must be a serious matter (See the Ten Commandments section of the CCC.)

If one of these criteria is missing, it is not a mortal sin.

As I said in another answer to a question:

When you fall, like I do, so many times, just get back up and, if it is a serious sin, go to Confession first, before receiving Communion at Sunday Mass and if you are not properly disposed to receive the Blessed Sacrament on any given Sunday, say a Spiritual Communion.

If one is having trouble discerning mortal sin from venial sin, they should consider finding a good, faithful Spiritual director who they can go to for guidance and advice.

I hope this helps,

Mike