It sure ain't good for a couple reasons. With regards to passing anything, it actually makes it more difficult. Muscles get larger when worked and pumped up with blood, so you are actually causing additional narrowing, therefore making the process more of a challenge.
Secondly, it definitely increases blood pressure. If you are younger and have a healthy circulatory system, not so much of an issue. If you do have circulatory issues, that can cause real problems.
Best thing, although difficult, is actively try to relax. Put something under your feet to get a better angle from your thighs, it should help ease things a little. My GI doctor has advised me to take some mineral oil when I have similar issues - it will lubricate without creating the "need to push" feel that some bowel treatments have.
If you have an upcoming appointment with your doctor, let him/her know. It could be a sign of a stricture in your rectum - I have this pleasure. It can be treated with a balloon dilatation. If it continues or gets worse, definitely call...