I am 19 years old, I used to weigh 148, I'm 5'5".

Two months ago I went to the gym and dieted while exercising, doing at least 45 minutes of cardio a day. I dropped down to about 138. I would eat about 2500 calories and I took 1 protein shake each time I worked out (about 4 times a week) I notice that my pant size has dramatically decreased as now I can't walk around without fearing that the pants will fall down.

I want to look like a Spartan before March. Should I abandon trying to lose weight and drink 2 protein shakes (1 scoup each drink) and eat regularly so that I can gain some muscle? Your input and help is greatly appreciated.

you wont gain muscle by doing cardio,even HIIT will only give you a limited amount of muscle,you need to do weight training.

I'm sorry I forgot to add that I lift weights as well.

you need to bulk,to build muscle ,then cut to show it.
so you need to eat more now then cut when you have gained enough muscle,dont go crazy with the diet though eat clean and up the cals gradually.

I understood everything except for cut. What do you mean by cut? Did you mean to get cut (ripped?) How would I go about getting ripped after I get the muscle?

Getting so-called ripped, is a function of low body fat, brought mainly through knowing your MT line, and backing off a healthy margin and coupled with overall weight training (and cardio if one wishes) to compliment this deficit diet. One simply has to educate and learn the APPROXIMATE rate their body is burning calories in relation to their coupled activity level (all of it). Of course eating clean, and having cast iron will power especially when the body fat starts to drop to low levels as the body will begin some serious resistance.

as chillen said.
a cut basicly means cutting cals or dieting alongside training.

My mother makes all my food so I can't really keep a calorie count XD. How do I check my diet then?