Take some REAL voice lessons to learn how to get the best from your voice. Voice category CANNOT be determined by range alone, especially since there is a big difference between being able to easily SING notes in an actual song, and just squeaking, screaming, grunting, or growling them out. Vocal technique is also a big determination of how easily you can sing in a given range, and how you sound when you sing it. You may be most "comfortable" singing in a rather low range because you may simply PREFER to sing in a pure chest voice, and D5 is a high as you can manage to push before your voice before it cracks, or you HAVE to shift to a different register. Since so many popular singers today are "belters", even if they rely on actual screaming to get the "note" out, many young fans think that is the only way to sing. Using head voice (sometimes incorrectly labelled as "falsetto" even by young girls who ahould be NATURALLY sounding that way ), is thought as cheating by inferior singers, or something used only by "opera singers".
There are a lot of young, untrained sopranos going about thinking they are "altos", because they can't or won't effectively use their upper registers which in the long run does cause vocal loss and damage. Even IF you are a true contralto, to sing even basic choral music (and certainly for any classical, Broadway, and anything but the simplesr of songs) you NEED to be able to sing a good, solid E5 here and there. That's an octave LOWER than the highest "note" you reported, so there IS something seriously wrong with the WAY you sing if an E5 is not in your "comfortable range".
If, when you do go higher, and HAVE to "shift gears" sorta speak, if it sounds like another girl has taken over singing, rather than you singing higher, then again, vocal training will make transitioning up and down smoother. Proper training with the correct teacher can also help you AVOID that "opera" sound if you hate it and aren't interested remotely in classical or theatrical repertoire, but still let you avoid strain
One other thing, as an 18 year old, you are at an ideal age for formal training, but your singing voice is far from mature. Any vocal classification that is appropriate now, may be drastically different (or not) by the time you reach your late 20s or early 30s. Voices can and do change in range and timbre just through natural maturation.