The only way to alleviate your concerns is to make an appointment and see your doctor so that he can examine the hard lump and discuss your problems with you.
I am sure that you have nothing to worry about. It is always best to see a doctor if you are worried about something.
If it was a sebaceous cyst, there are chances of it forming again over a period of time. since you say that it has formed again and is hard, what you can do is apply heat to that area to make it go away. Usually, when you get such cysts removed, sometimes they are fragmented and arent completely removed and then they reappear. try the heat application and see if that works for you, it worked for me. You can also apply warm water directly to that area while taking a shower.
Some people are prone to getting (scar tissue like raised bumps) called Dermatofibroma. They are benign and usually caused by any kind of over irritation of a hair follicle (can also occur when kids have chicken pox not that children get that anymore these days) ie rubbing of pant legs, bra straps, undergarments, headbands, etc.... forms a hard nodule/cyst that is mainly a form of scar tissue. It may have started as an ingrown hair/pimple/chickenpox/sometimes even irriation when you have 'goosebumps' etc. It turns into a form of scar tissue. If you get it removed, it may come back (due to the fact that it is in fact scar tissue in the first place. It may return larger in diameter due to the removal procedures need to remove a bit more than just the effected area.
I suggest showing your doctor to confirm. But I believe it was a dermatofibroma that your doctor should have told you may not be fixed with removal. Sometimes removal of a larger raised (scar tissue)nodule creates a flat nodule(scar tissue). If what I'm thinking it is is correct, then this is completely normal. I would definitely speak to your doctor or a dermatologist to confirm this.