My girlfriend and I had protected sex with a condom that broke. I didn't finish inside her but there was some pre-. That was 3 moths ago. Since then she had 3 normal periods (in length and flow) and done 3 HPT. All HPT were negative (last one was taken about 2 weeks ago). In past 2 weeks I have noticed that she has gained some weight and that she is eating a lot. There are some skin marks on her back (but she says it's from sun). Also her boobs looks and feels bigger (but she just got off period). Can she be pregnant despite having a normal period and negative HPT?

No, she could not be pregnant from that sex time. If you have had other sex, (even with a condom) there are many condom failures that can not be seen. Condoms have a 3 to 18 percent failure rate.

So if you had sex again, last month for example, she could be pregnant from that.

My girlfriend and I had protected sex with a condom that broke. I didn't finish inside her but there was some pre-. That was 3 moths ago. Since then she had 3 normal periods (in length and flow) and done 3 HPT. All HPT were negative (last one was taken about 2 weeks ago). In past 2 weeks I have noticed that she has gained some weight and that she is eating a lot. There are some skin marks on her back (but she says it's from sun). Also her boobs looks and feels bigger (but she just got off period). Can she be pregnant despite having a normal period and negative HPT?

To be simple. No. A normal period indicates no pregnancy. Some women will bleed while pregnant and for the first little bit assume that this is their period. The chances of being pregnant though, considering the several negative HPTs is almost nil.

To be blunt. She's just gaining weight. It happens to all of us.

FYI: Women's bodies change CONSTANTLY. Each month we gain and loose weight and breasts feel and look differently.

I may be wrong, but you sound very young and inexperienced. A period would almost certainly mean she's not pregnant. A negative HPT on top of that is further proof. Three months passing with both period and negative HPT is a definite sign she's not pregnant.

Consider more reliable birth control, such as oral contraceptives in addition to condoms and stop using HPTs unless she actually misses a period