The fact you're on here should be enough to drive you to the gp. If you thought it was worth ignoring, you'd not seek out a cancer forum. So, the worst case scenario is already going through your head. Fear or ignoring a lump doesn't cure any cancer, but both mean the difference between a heap of treatment or no treatment being offered as things are too advanced.

Lumps disappear, and appear due to positional changes, gravity etc, so the fact you can't feel it at times isn't an indication of it being good or bad. Cancer doesn't tend to be sore. Cancer only hurts when it's pressing on nerves, against a structure that has nerves, causes a blockage or stretches a lining that has nerves. In fact, if it did hurt on its own, that would more likely indicate an infection of some sort.

If my wife didn't want to bother her gp or assumed her lump was nothing, she'd have been getting told her care was palliative, not curative. So waiting or embarrassment doesn't achieve anything other than kicking the can further down the road. The chances of it being nothing (cyst, lipoma etc) are statistically in your favour.

No one can tell you over the net if your lump is harmless or not. People will have had the exact same experience as yourself and things will have been ok, whilst others will have been diagnosed with cancer with those very symptoms you have. The only way a lump can be deemed harmless is having a gp seek the lump out and physically examine it. If s/he fails to find the lump, and you are adamant one is present, they tend to send you for an ultrasound which can pick up hidden lumps in the breast. If the GP isn't sure what it is, you will be sent for further tests to prove one way or the other that your lump is benign.

Edit: 1.5cm is quite a biggie. not massive, but not small either.

Males do need to get lumps checked out. I know because I had to get a male to go and get lump checked and the male really didn't want to do so. I had a word with the nurse doing my own (routine) mammogram and it turned out a male in her family had been affected. So, I definitely asked a good person for advice. It's always better safe than sorry. GPs are really good at doing checks and won't find it at all odd your asking to be seen. Even if they don't refer you, they should tell you how to monitor things and may even ask to check you over again in a few weeks. Do make sure you tell them all the details you put down in your posting here. Good luck!